NetHack/Staying Alive
NetHack is not Quake.
Always remember that NetHack is a turn-based game. You have as long as you need to consider your options, even in the heat of combat. In fact, when you find yourself in a sticky situation, it is wise to walk away from the keyboard while you think about what to do next. You could even save your game and consult the spoilers or the NetHack newsgroup.
The better part of valor is discretion.
In NetHack, there are no prizes for dying with honour. When a monster is giving you a hard time, it is seldom wise to take one last swing at it in the hope that it will die. It is much better to escape from the situation and return when you are more able to handle it. One reason why experienced players are fond of speed boots is that they enable you to run faster than most monsters.
Running is not the only way of escaping a dangerous situation. It's a good idea to carry a scroll or wand of teleportation for emergencies. Most monsters won't follow you up or down stairs, though a few will. If you have a wand of digging, you can use it to dig a hole through the floor, though again, some monsters will follow you. A cursed potion of gain level will cause you to rise up through the ceiling.
If you can't (or don't want to) flee, you could try to heal yourself. Prayer is effective if you are in reasonable standing with your deity, you haven't pestered them for help too recently, and they recognise that you are in need of healing. Their definition of "in need of healing" is either 5 HP or less, or 1/7 or less of your maximum HP. If you don't meet this definition, try some other means of healing, such as spells or potions.
An alternative survival strategy is to inscribe the word "Elbereth" on the ground. This prevents most non-human monsters from attacking you hand-to-hand, though they can still use ranged weapons, wands and spells. You can scribble "Elbereth" in the dust with your fingers, but then it is easy to erode, so it is better to engrave it with a wand of digging, and even better to burn it with a wand of fire or lightning. For full details on how Elbereth works, see Kate Nepveu's Elbereth FAQ.
If none of the above applies, and you feel you have to keep fighting, bring out the magic items or spells rather than hacking away with your weapon. Rays bounce, so take care to avoid being killed on the rebound, for example by choosing a wand to which you are already resistant.
Sometimes an indirect approach is best.
Walking right up to a monster and whacking it with your melee weapon is not always the best approach. Some monsters, such as mumakil, hit hard but are slow enough that you should be able to keep your distance and kill them with ranged attacks. Others, such as floating eyes, are totally harmless until you hit them in melee, but have a nasty counterattack (in this case, they paralyse you with their gaze, leaving you vulnerable to any newt that may appear). Pay attention to monsters that it is unwise to attack in melee, and think about alternative strategies for handling them. If you don't have a good ranged attack (like a bow and arrows or a powerful wand or spell), then throw junk weapons, or even rocks.
Newbie mistakes to avoid
editDo not savescum, reroll, or use similar dodgy tactics.
Savescumming is the practice of making backup copies of your save files and restoring them when you die. Since this involves going behind the game's back, it is universally regarded as cheating. No-one will be at all impressed if you ascend by savescumming, especially not if you try to pretend that it was a genuine ascension.
Some people savescum in order to learn the game. This is harmless provided that you do not boast about savescummed games as if they were genuine and that you play on a single-user machine (you won't be able to savescum on a multi-user machine anyway). Even so, explore mode is a better option because it does not clutter up the high score table with scores obtained by cheating.
Savescumming and explore mode are generally considered to be a less effective way of learning than playing the game properly. When you can refuse to die, or save your game before a difficult encounter and restore it if you get killed, it is tempting to just keep trying the same strategy until you happen to survive, rather than thinking about ways to improve your chances.
Incidentally, it should not be necessary to back up save files as protection against your computer crashing. NetHack comes with a "recover" command for restoring crashed games.
Rerolling means that if you don't like the stats or items which your character starts with, you immediately quit and try again. Although this is a legitimate part of some other games (for example Angband) it is generally considered to be abusive in NetHack. Like savescumming (though to a lesser extent) rerolling is an ineffective way to learn the game - the ability to keep weak characters alive and to make do with the equipment you find is an important part of being a skilled NetHacker.
Many players who do not savescum or reroll will deliberately play in a risky manner in the early game, because you haven't lost much time if a beginning character dies. For example, they might drink from fountains hoping for a grateful water demon to grant them a wish, even though there is an 80% chance that any water demon they get will be hostile and will kill them. This is not considered as bad as savescumming or rerolling, but is still frowned upon by many NetHackers, who feel that you should try your best to ascend each character. Also, when you play this way you miss out on the valuable experience of struggling to keep a marginally viable character alive for as long as possible.
Do not attack peaceful monsters.
Some monsters will be described as "peaceful" when you look at them, and you will be asked to confirm any attempts to attack them. It is generally a good idea to leave them alone, for two reasons. One is that the gods disapprove of killing peacefuls, and will exact an alignment penalty, along with more severe penalties for killing a peaceful human (unless you are chaotic) or a unicorn of your own alignment. More details of the penalties for killing peaceful monsters can be found on this page of myths and facts about NetHack. Note, though, that there is absolutely no penalty for allowing your pets to attack peaceful monsters, or even for healing them as they do. Nor will the monsters retaliate against you for your pets' actions.
The other reason to leave peaceful monsters alone is that many of them are much stronger than you. In particular, tangling with a priest or shopkeeper is likely to lead to your death. (You can use a stethoscope to check how strong a monster is.)
Only pray if you have good reason to.
Some newbies feel that they "ought" to pray on a regular basis in order to keep in touch with their god. In fact, prayer in NetHack is always pestering the gods for help, never praising or thanking them. Hence your god does not mind in the least if you never pray, but does object if you pray too often. Once you get a negative response to a prayer, you will almost always be unable to pray until you have placated the angry deity with a suitable sacrifice.
However, prayer can save your life in an emergency. Just remember that your god's definition of an emergency may not be the same as yours. Wounds aren't an emergency until you are either at 5 HP or less or at 1/7 max HP or less, and hunger isn't an emergency until you are weak from hunger. The praying spoiler gives much more detail about prayer.
Eat only fresh corpses.
Most corpses are safe to eat when fresh, though some are poisonous (see the Food section later for specific corpses which are particularly good or bad to eat). Old corpses, though, will make you fatally ill (and have no nutritional value either). There are a few ways to cure this deadly food poisoning, but it's best to avoid it, by not eating anything that you didn't kill yourself or see die. Since zombies and mummies are created by magically animating an old and rotting corpse, undead corpses are never safe to eat.
Don't eat while satiated.
If you eat while satiated, there is a risk of choking over your food and immediately dying. Eating once you have reached the point of satiety at which the game tells you "you're having a hard time getting all of it down" is particularly dangerous. Experienced players may be able to tell when they can safely eat despite being satiated, but it is safest for novices not to try it. An amulet of magical breathing will eliminate the risk of choking, and a tinning kit or an ice box will keep food fresh for later.
(As long as you are not satiated when you start eating, you should always be permitted to finish your meal. However, owing to a bug this may not apply if you save the game in the middle of a meal. At least one character has died from this.)