Texas Hold'em is played in the main event of the World Series of Poker, and is considered by many the most complex and strategic form of poker. In the game everyone tries to make the best 5 card hand they can out of five community cards and two personal cards. In modern games the blinds are then posted. The blinds are forced bets. The player to the left of the dealer puts in the small blind, and the player to the left of the small blind pays the big blind, which is typically twice the small blind. This represents the minimum bet once the action begins. Each player is dealt two cards face down called the hole cards, which they aim to keep secret. Then the first round of betting ensues, with the player to the left of the big blind acting first. After this, a card is discarded and three cards are dealt face up in the middle of the table. These cards are called the flop, and are the first three community cards. There is another round of betting, beginning this time with the small blind. One more card is discarded and one more community card is dealt, called the turn. There is another round of betting, again starting with the small blind, then a discard, and then the final card, called the river, is dealt. There is one final round of betting and then the players who have not folded turn over their cards.
Basic Strategy of Texas Holdem
Note this strategy guide is for playing no limit texas holdem. Slightly different considerations apply when playing limit or pot limit holdem. It is also heavily oriented towards tournament play, since this is the format where no-limit is most commonly played.
Position refers to how many players must act after your actions. It is generally preferable to be the last player to act, since your opponents will often reveal some information about their hand by betting or checking (sometimes they may be trapping you by slowplaying or checkraising) before you act, because you have the choice of taking a free card if no-one else has bet, and for various other reasons. Position is probably the single most important factor in no-limit hold'em. The person acting as dealer (also called "being on the button") will always be last to act (except in the first round), and is thus said to "have position" on all other players. Because the dealer can see what all the other players do it is the most desirable position to occupy post-flop.
The blinds, the button and the cutoff seat (one off the button) are normally referred to as late position; the next two or three seats are middle position; and everything else is early position. One important thing for a beginner to realize is that the playability of a hand varies greatly according to their position. Beginners should aim to play fewer than 10% of hands in early position (when you are one of the first to act) but might find 30% playable in late position in an unraised pot.
It is useful to have a shorthand for the pre-flop positions at the table. For this guide, the big blind will be denoted by BB, the small blind by SB; other positions will be represented by a number, with 0 as the button, 1 as the seat next to the button, 2 being 2 places to the right of the button and so on.
Principles of Starting Hand SelectionEdit
The single most important thing in choosing which hands to play is to avoid being dominated.
A hand is dominated if it is a non-pair hand facing a higher hand with one card the same, or facing a pair of the same or higher value as one of its cards. For example, AQ is dominated by AK, QQ, KK and AA. QJ, which looks like a good hand to beginners, is dominated by AQ, AJ, KQ, KJ, AA, KK, QQ and JJ - that is, virtually any hand playable for a raise but AK and the medium pairs. A pair is dominated by any higher pair.
This is important not just because the dominated hand is, at best, a 3 to 1 underdog. The implied losses involved in holding AQ against AK are huge, since it can result in losing a large portion of your stack when an ace flops. The legendary player Doyle Brunson once gave up playing AQ entirely, saying that it had lost him more money than any other hand.
This leads to the Gap Concept - you need a better hand to call with than to raise with, from any given position. A player on the dealer button may raise with KJ after the action has been folded around, but should normally fold it to a raise which they judge may represent AK, AJ or KQ. More on this later.
Types of handsEdit
Hands can be divided into a number of types. The value of the types changes depending on the number of players, the level of the blinds and the player's position on the table.
Some hands here are chosen for their ability to make a useful strong one-pair hand; others are chosen for the ability to make a straight, flush or set. The latter are known as drawing hands. A good general rule for drawing hands is: play them in later positions, perhaps the button and positions 1 and 2, and never call with them for more than 5% of the maximum possible return. The maximum return is, roughly, the minimum of your stack and the opponents stack. You might well invest more than that when raising, of course, if you have a stealing opportunity.
A few playable hands, which are not mentioned, fall somewhere in between these two - K9 suited, for example, has a value perhaps somewhere close to KJ off. (But it should be noted that KJ off is not nearly such a strong hand as most beginners would tend to think.)
Big pairs (AA KK QQ)Edit
These cards are a favourite against any hand (except a higher big pair) and must be played aggressively preflop. The general idea is to make as big a pot as possible with as few opponents as possible - which means that you should very rarely slowplay these hands, unless you are playing opponents who know your style, in which case occasional slowplaying has some value. However, there are three general rules for these hands in small tournaments: raise, reraise and all in.
Post-flop, by contrast, pays to be careful. If the pot is small relative to your stack, you are probably only going to find all of your chips in the middle when you are beaten - after all, all you normally have is a one pair hand. The main worries are overcards, a high pair on the board, or a flop with three cards to a straight or flush, especially a high straight like Q-J-T. Knowing when to push for maximum value against someone with top pair or a smaller overpair, when to make a player pay for a draw, and when to slow down or throw your hand away is a question of experience.
High cards (AK AQ AJ KQ AT KJ QJ KT)Edit
Post flop, the hope with these hands is to make top pair, ideally with top kicker. For example, if you play AJ and the flop comes J 8 4 of 3 different suits then you are leading against most hands, such as KJ and QJ. By raising preflop you should eliminate hands such as J8 and 44, or at least make them pay exorbitantly to hit their hand, and so you only need to worry about a player holding 88, JJ or higher. It is standard practice to bet strongly with top pair here, to make it unprofitable for an opponent with T9 to draw for a straight, and avoid someone hitting an overpair. T.J. Cloutier has said that 'Hold'em is a game of top pair, top kicker.'
Ace-King should be regarded in a different category from the other hands, since every time it hits it makes top pair, top kicker with minimal possibility of an overpair. The other hands should be played much more carefully, especially in a raised pot, and especially in a full ring game, where a dominating hand is present much more often than you might think.
Small and medium suited connectors (JT T9 98 … 43; QT J9 … 53; Q9, all suited)Edit
Numerically adjacent cards of the same suit are less likely to make a playable hand on the flop than two big cards. However, when they do make a hand it is usually a very strong one such as a flush, straight, two pair or trips. While pot odds rarely favour these hands, no limit holdem is a game of implied odds, and making that type of hand can easily allow you to double through. Against good players they have the additional advantage of making your play less predictable.
With suited connectors the main aim is to enter a pot cheaply against a number of opponents and from a good position, usually either on or one off the button. With an open-ended straight or flush draw the player will often make a bet called a semi-bluff. If they are called then they are probably behind, but on drawing the correct card it may be possible to force a player with a hand as weak as top pair-top kicker to move all in.
Suited connectors have the most value in full ring games and with low blinds. The true connectors, with no gap between them, are more valuable because they are more likely to make a straight. However the one-gap connectors sometimes gain value from being more deceptive. The medium, offsuit connectors such as JT and T9 may also be playable in late position. The smaller offsuit connectors are trash hands from any position.
The medium connectors are excellent hands to find in late position in an unraised pot, since many players are often holding lower connectors, small pairs, and (especially in online play) high-low combinations, where the medium connectors have a distinct edge. In raised pots the small connectors may become more valuable than the medium ones, however. For example, if a player raises with AQ and another calls with AK, then a 65 suited will be quite a playable hand in a showdown, since the AQ is such a big underdog. (There are pathological cases where the 65 can actually be a favourite in a showdown over the AK here.) Better yet, a flop like A-K-6 will not lose many chips for the player with 65... while a flop like K-6-5 could be quite expensive for the player with AK. By contrast, a QT would be a very bad hand to have.
Suited rag aces (Ax suited)Edit
The ace with a low suited kicker plays very similarly to suited connectors. While pairing the ace is useful, it can easily cause problems when outkicked by a dominating premium ace, and should be played more like middle pair than top pair. The idea, however, is to flop a flush draw. While many inexperienced players will play any two suited cards in the hope of this happening, suited aces have unique advantages here.
Most obviously, when making an ace-high flush you may have the nuts, and you will break any player with a lower flush. Also, when the flop brings a flush draw there are two scenarios; there is no ace, in which case you have a flush draw and an overcard, or there is an ace, in which case you have top pair with a flush draw. This means you have 12 cards to improve your hand above the current top pair by making a flush, pair of aces, or two pair. This gives a very good chance of winning a showdown. Unfortunately, the flush is so obvious that it will take very good play to extract maximum value from a solid, conservative player.
It is worth bearing in mind that an ace with a wheel card (2, 3, 4 or 5) is a stronger drawing hand than an A6 or A7. You may flop a straight, or a flush draw with an inside straight draw, a genuine monster draw with 15 outs over a pair of kings.
Small and medium pairs (JJ TT .. 22)Edit
The medium pairs, JJ through 88, are often the hardest hands to play. For example, suppose a player raised in early position and you called with 99. The flop comes 8-5-3, and the opponent bets. It is clearly unlikely that they have paired the flop, since they raised; if they have a pocket pair of eights then they have a set; if they have a higher pair then you are a massive underdog. All you can beat is a bluff, and even against two overcards you are only a 3-to-1 favourite. So, the medium pairs require very careful handling after the flop unless you hit a set. At the same time they are a little too good to throw away to a single raise, and certainly good enough to raise with. In fact they may be re-raising hands against an aggressive player.
With lower pairs, 77 through 22, hitting an overpair is increasingly unlikely, and they are played more as drawing hands. The standard plan is to enter the pot as cheaply as possible and with good position, try to make a set on the flop, and release the hand upon failing to hit. The pair will make a set approximately 12% of the time, which is enough to pay off handsomely, especially since it is nearly impossible to tell whether or not someone has made a set from the cards on the board.
Trash hands (Almost everything else)Edit
Trash hands are trash not because they cannot win - any two cards can win, especially in a game where you normally do not show down a hand - but because it is so difficult to make a hand which you can play strongly on the flop. Take K7 off suit. When pairing the king it cannot really be played as top pair because of the poor kicker. When pairing the 7 it is unlikely to be top pair, and also unlikely to remain as top pair once the turn and river have been dealt. Making two pair can be very profitable, but with only a few percent chance of flopping two pair even the implied odds cannot justify the play.
As a result, trash hands are only playable under certain circumstances. One is a common scenario at the final table of tournaments. The blinds are high, giving you little chance to select premium hands, and the table is shorthanded. This forces gambling play. By the time a tournament gets down to two players it may be normal to go all-in on being dealt a K7. They are also playable for a call or check from the blinds in an unraised pot.
It is also possible to play them simply because your position at the table makes playing any two cards advantageous. This normally occurs when you are the dealer and the players in front of you have passed. If you are an experienced player or have weak or conservative opponents in the blinds then it can be profitable to play simply for the opportunity to bluff.
Outside the blinds, it is vital to raise when playing these hands preflop, not call. By raising you give yourself the opportunity to steal the blinds as well as the best chance of finding yourself heads-up, where it is easiest to win with the worst hand. Also, limping in where you cannot call a raise is rarely a good strategy. This approach works for some of the best players in the modern game such as Daniel Negranu and Gus Hansen, but it is extremely difficult for inexperienced players, who should normally stick to more playable hands.
There is a wide range of pre-flop tactics aimed at winning the pot outright or at least narrowing the range of remaining players before the flop.
The Button StealEdit
While any raise preflop can be seen as an attempt to win the blinds, there is one scenario where it is especially profitable. This is the previously mentioned situation of having every player before you pass when you are on the button. If the small and big blind will both fold to a raise of twice the big blind two times out of three then the third time is effectively a freeroll, and a freeroll with good position. Late game in high level tournaments is dominated by attempts to steal the blinds in this fashion. Of course, it is possible to steal from positions other than the button, but having late position is very desirable.
The squeeze is best demonstrated by example. Suppose a player raised and was called in front of you. A reraise here will place the original raiser in a difficult situation. They have no way of knowing what the original caller will do, and any chips they place in the pot may have to be surrendered to an all-in by a large pair or similar. As a result, a good player will not call without a huge hand. This move almost always either wins the pot outright, or sweetens it and moves to heads-up. Even some conservative players will make this move with marginal hands such as suited connectors and small pairs.
This tactic is exactly what it sounds like - you limp into the pot, an opponent raises, and you make a big reraise. This tactic is somewhat frequently used by novice players when the are dealt AA or perhaps KK from an early position. However, a thinking player could use this play in a variety of situations. For example, let's say that you are dealt JT suited from a relatively early position, you limp in, another player limps in, and an aggressive player puts in a small to medium sized raise from the button. If the blinds fold, you should occasionally raise in this spot. If you do raise, the player who limped after you will almost always fold, and since the button is aggressive and has position, he could easily have a semi-strong hand that he is now forced to fold. Even if he does call, you still have a disguised holding plus you will get first shot at the pot on the flop. Three things to understand: Only use this play on occasion; do not restrict this play to the times when you have a huge hand; try to use this play only against players who may raise with marginal holdings.
Stealing from the BlindsEdit
This is another play that is best illustrated by example: A few players limp into the pot; you put in a big raise from the small blind or big blind; everyone folds and you scoop the pot. This play actually works more often than it sounds like it should. The best circumstances are when two or three players who like to limp with many different hands enter the pot from middle position and late position, AND when you have a hand that is completely worthless. The play is wasted when you have a hand that you would like to see a cheap flop with, which may develop into something big (like a suited connector or small pair). Note that this play also disguises your raises with very good hands. If the only time that you raise is when you have a big pair or a big A-high, you will be too easily read by your opponents.
Post Flop PlayEdit
Reasons to Bet, Check and CallEdit
One of the main mistakes beginners make is acting without a reason. Or rather, acting with a reason that has to do with their cards rather than with gaining chips.
The main reasons to bet and raise are:
- To gain value
This is the simplest reason. You have a hand, and you think that your opponent may have a worse hand with which they can pay you off. Value bets are the heart of poker.
- To force a better hand to fold
Bluffing, discussed in detail later. Your chances of winning a showdown are slim, so avoid one.
- To protect a hand
Almost any hand can be outdrawn, and it is often important to force your opponent to pay for the privilege. How important this is depends on your hand, the board and your opponent's range of hands.
- To gain information
Betting and raising purely for information is a technical mistake - you are betting in a situation where the bet will not generate income. However, it may be correct to make a small mistake to avoid a large one later.
The main reasons to check are to:
- Induce a bluff or misrepresent your hand
This might be slowplaying, or it might not in a normal sense; you might check a middle pair to induce a bluff.
- Keep the pot small
If it's not clear whether you are ahead or behind, or are 1 card away from making your hand, why actively build a bigger pot? Checking a hand down might be a good option in this case because you can bow out if someone (or a few) raise. Also, having more players in the hand means that your bets vs the total pot are less costly vs the odds of improving your hand.
- Take a free card
In position, especially against a tricky, trapsetting opponent, you might be better off seeing if the next card improves your hand when they check to you. Out of position you cannot guarantee a free card, of course.
- Gain information
Sometimes, against certain opponents, you might gain more information by checking than by betting. Be aware of these times.
The reasons for calling might be related to a number of the above - but generally, call when neither folding or raising seems like a good option.
These may seem like simple principles, but balancing them in the many different situations of hold'em is one of the keys to post-flop play.
Evaluating the Strength of a HandEdit
There are several factors which affect how strong a hand is besides its face value.
The more superior hands which can be made or drawn to from the flop, the weaker the hand. Two black aces are excellent preflop, but on a flop of Q J T suited in hearts it becomes a very mediocre holding against the possible flush, straight, two-pair, set and straight flush combinations.
More players in a pot also weakens a hand. Top pair jacks, ace kicker may be excellent heads-up, but it is far less likely to stand up with 5 players, and almost dead if those players are allowed to remain for the turn and river.
A hand becomes stronger when the amount in the pot is high in relation to the maximum you can lose. All-in with top pair is normal when the pot is similar in size to your stack, but suicidal with a ratio of 20:1.
Also, in tournaments the size of the blinds in relation to your stack is an important factor. A shortstack must judge whether it is worth waiting for a better opportunity to double through with a lower stack, or worth making a move now with a weaker hand.
Evaluating the Strength of an Opponent's HandEdit
The first thing to do on the flop is to evaluate whether the opponent has made a hand. As a rule of thumb, remember this: against one opponent, you will be correct in assuming they missed the flop two times out of three. If the flop has a pair, it is even more likely that the flop did not help their hand. Against more than one opponent, you will be wrong two times out of three or more.
Position is the most useful tool in judging their strength. If they are forced to act before you and check, and if they have no history of check-raising or slowplaying, then it is safe to assume that they hold no hand. This is not necessarily true if the flop came A K Q -- they may be worried about your hand, or they may have flopped the nuts.
On the other hand, with a flop like 7 5 2, a bet may not mean much, especially if the opponent considers you a tight player. This type of bluff on a rag flop is so common as to hardly be considered a bluff; they may be simply protecting an ace-high.
The game of poker cannot be played correctly against rational opponents without bluffing. Of course, not all opponents are rational. If you find an opponent who invariably calls any reasonable bet on the flop and beyond, then only bet if you have a strong hand -- this sounds obvious, but it is something so alien to many players that world champions have been known to struggle in small stake, friendly games.
There are 3 ways in which bluffing pays off:
- The opponent folds.
- The opponent calls, and you then draw out to a stronger hand against them.
- The opponent calls with a stronger hand, sees the bluff, you lose, but the opponent is more inclined to call bets in future because of it.
Against good opponents it may be that the last of these is actually the most profitable.
It is very difficult to establish how often you should bluff. Mathematically, a bet of half the pot breaks even if the opponent(s) fold one time in three, and a pot-sized bet breaks even if they fold half the time, based only on the possibility of them folding. Heads up against a conservative, inexperienced player it may well be profitable to make the first bet on the flop regardless of your hand. It certainly simplifies decision-making.
The main cost of bluffing on the flop is probably the opportunity cost of slowplaying. Checking with a very strong hand is something of a tell if you have bet the last 40 flops.
Slowplaying, or trapping, is one of the most dangerous moves in Hold'em. The principle is to check after making a good hand, allowing the opponent to either bluff at the pot or improve their hand slightly on the next card.
The problem is that the opponent may find themselves drawing to a better hand than yours. Unless you are able to make some painful laydowns it is important to only trap with hands which are quite unlikely to be outdrawn, given the texture of the flop.
Immediate winners (also known as "monsters") include nut hands that are impossible to beat (any four of a kind or better, any nut straight or better). With these hands, the imperative is to make as much money as possible. Betting big will scare most players out of the game with many of them guessing your hand, so slowplaying all the way to the showdown is advised. Call very often, but make middle-sized bets if nobody else is inclined to bet. Loose players will often follow you with draws and weaker made hands. You should almost never raise with immediate winners unless you're sure the opponents will call, since your goal is to maximize your winnings. These hands are virtual cheques for the pot and you should almost never fast-play them. Note: beware the nut straight, a connected card on the turn can re-position the nuts!
Non-nut winners include non-nut straight or better. These hands include:
- full houses with xxy-shaped flop and lower-ranking card making your trips — e.g. flop: 99J, hand: J9 (nut: JJ)
- full houses with xxx-shaped flop and KK or lower in the hole — e.g. flop: TTT, hand: QQ (nut: Tx)
- flushes where you don't have the highest card in the suit not on the table present in your hand — e.g. flop: A72, hand: QJ (king is missing!)
- outside straights where you don't have the next two cards higher-ranked than three on the table — e.g. flop: QJT, hand: K9 (nut: AK)
- gapped straights where you don't have the gap card and the higher next higher-ranked card — e.g. flop: 986, hand: 75 (nut: T7)
With non-nut winners you have two strategies available: either scare the opponents outright by making large bets or delay that action until the turn. Should a player follow, be prepared to back your hand to the all-in. You should use the first strategy when there are plenty of scare cards possible on the turn and the possibility of completed bigger draws is higher. The second strategy is advisable if the turn doesn't pose a danger. In both cases you must know when to fold.
Three of a KindEdit
Sets are trips where you have the pair in you hand (a pocket pair). The top set is the set where your pocket pair pairs the highest card in the flop. Likewise, at the middle set the middle card is paired and at the lower set the lower card. All sets are very good hands, but seeing the turn is something you want to avoid, since you only have one nut out and many more cards will be scare cards. Nevertheless, a set can be a good hand at the showdown, but it's way more profitable to get out of trouble before the turn. Make large bets (even doubling the pot is okay) to push the opponents out of the pot. Alternatively, if the board doesn't have consecutive cards or cards of same color (unconnected), you can try slow-playing and make people with weaker but still good hands stay in. A set is a great hand, but a low set has to be protected. You should play more passively with top and middle sets, but don't let your betting become predictable.
Flopped trips is trips where you have one card in the hole and two in the flop. It is similar to a set, but not as good. Trips are usually slow-played so as not to scare everyone out of the game. (Usually, if you encounter a paired flop, you should be suspicious of a full house or a four of a kind. In this case you should be looking for these hands in other players.) But if the board is connected, be aggressive.
For the purpose of determining the flop strategy, a two pair requires that you have one card of each pair in the hand and one on the table. Two pair where you have a pocket pair and the board is paired is not a good hand and you should never count paired flops toward your two pair.
Top two pair and "rags & riches" two pairEdit
The top two pair has both top and middle flop card paired with the hole, while the "rags & riches" two pair has the top and bottom card paired. Both are to be slow-played under ideal conditions (uncoordinated board, no hint about immediate winners available from other players). If you see a possible straight or a flush on the board, bet high and raise to scare off drawing hands. Two pair is good, but you're in fact drawing to a full house. You may stay to the showdown, but make sure in most cases you'll be beat by then.
Bottom two pairEdit
Since the cards you play that are low enough to make the bottom two pair possible, are usually connected, you'll get dangerous flops. Make sure the drawers get out before the turn. There's also the possibility of top card being paired and thus making everyone a top pair and degrading your hand to the level of middle pair, low kicker. This advice is also valid for the "rags & riches" two pair.
Pocket pairs are pairs where both cards are in the hole.
Overpair is a pocket pair bigger than the top card in the flop. When you have the biggest overpairs (pocket aces or kings), slow-play and pretend you bluffed. If you're against multiple opponents, try to filter them out, since the value of an overpair decreases with each new community card. If the overpair isn't that big (usually TT-QQ), bet more aggressively since your overpair isn't that good anymore. Be wary of flops with straight and/or flush draws. If someone chases you persistently at such a flop, you'll probably end up against a made hand or an even-money draw.
The 2nd pair is a pocket pair between the top card and the middle card. It is a usual bet/fold position. When in a late position and it's checked to you, take the free card. A strong player betting in front of you probably means he has something better than a 2nd pair. If you get raised after you've bet, fold unless you're absolutely sure it's a loose player drawing.
3rd pair is below the middle flop card. Check/fold or make a position bet if it's checked to you and the pair is above the bottom flop card. Do not call any bets or raises. This is a hand where you're hoping to get to the turn for free and get two pair or an unlikely trips.
Split pairs are pairs where you have one card in your hand and the other one in the flop. They aren't necessarily worse than pocket pairs, but the kicker is crucial.
Top pair, top kicker (sometimes known as TPTK) is the best split pair possible. If you don't see imminent danger (such as a connected flop), slow-play. Else, bet big and try to force everyone out. The only thing is that you have to make sure you don't get carried away by a leading bettor who has obviously beaten you, but you're still holding on to the TPTK. TPTK is not a miracle hand and definitely not an immediate winner. Any overpair or even two pair is way better and you shouldn't hold on to the old pair to the showdown if you don't improve your hand in the meantime. Beware overpairs if you don't have additional outs in your hand (e.g. straight or flush draw).
A middle pair is a pair involving the middle flop card. If you get a middle pair, you should either bet or check/fold, depending on the playing conditions. It is a big advantage here to have an ace kicker. With an Ax hand, you can play more freely and usually take a free card when possible, since you want to get two pair, trips or a flush draw. If you get two pair, you can dominate the overcards, if you get trips, you can dominate the other player with the trips by your ace kicker, if you get a flush, your ace will guarantee you a win etc. However, if you have a worse kicker, play less aggressively and don't take much free cards. Should a tight opponent bet, fold.
This is the worst split pair. Of course, the advice is to play very passively. If anyone bets, fold. In most cases you'll lose a bottom pair on the showdown, so your goal is to improve. You have only 5 outs to improve. If you decide to bluff and bet with a hand like this, do not make it a costly habit. If you're last to act and the table seems quiet, take a free card.
The opposite of made hands, draws are hands where you're aiming at a combination such as a straight or a flush, but you have only 4 cards necessary. You're aiming to get the fifth one either at turn or at river. If you have only 3 cards, you have a backdoor draw. The only feasible backdoor draw is a backdoor flush draw, getting you 1-2 outs. A backdoor straight draw is worthless. A nut draw is a draw to a nut hand.
Draws with 9 or more outs are generally good. Do not call here, make some action. 12 nut outs (ace-flush draw) are usually enough for a 50% turn & river chance, making you an even-money bet against a random opponent. The more opponents you have, the higher your calling odds are. You should remember not to "draw dead" (i.e. when you're drawing to a straight and it seems obvious someone has already got a flush). Also, beware of dead outs (your own outs that also help your opponents).
You should be more careful if you're not drawing to a nut hand. In that case, you could often by showdown find yourself against a better hand, even if you complete your draw. If you have a mid-to-late position and a rather good draw, try to make a middle-sized bet to take the pot outright.
If you have less than 9 outs, the pot odds become the imperative here. Do not call scare bets, you'll be wasting your money.
Big overcards are strong preflop holdings (AKs, AQs, AK, KQs, AQ, AJs) against a flop with no face cards with no made hand and no 4-card draws in sight. Be careful when playing with these against strong players as they know this is one of your most frequent hands. Sometimes try to pass it off as an overpair, but don't do it too frequently. Don't play this hand against more than two opponents.
Anything not mentioned here is usually not even worth a bluff. Check/fold as you'll usually lose with such bad hands. Playing something like this is OK only on a late stage of a tournament.