Harvesting marijuana is a joy and a blessing, until it becomes a pain in the arse. There is a great deal of work involved. Some strains will be more work than others but you can be certain that harvesting and trimming a crop will require a considerable time investment as well as sore hands and fingers.
When do I Harvest? edit
The best way to determine when to harvest is to examine the maturity of the trichomes. Trichomes are the resin glands of the plant and their level of maturity gives you an idea of both how mature the plant is and the effects you can expect from the plant if harvested at a given stage. In order to examine trichomes you will need a pocket microscope, these are commonly available for about $15 USD in 50-100x magnification, 50x should be plenty.
Trichomes are stalks with a head at the tip, it is the tips that you will look at. You will want to look at the trichomes in a couple areas over the plant to get a good idea of the overall maturity. The tips start out clear, then later will begin to cloud, finally the tips will turn amber. Clear trichs aren't very potent, the ratio of cloudy to amber is really what you want to look for. The more amber the trichomes the more of body physical stone the pot will deliver. A heavier ratio of cloudy will deliver more of a soaring mental high. All of this is of course relative to the general effects provided by the strain. Most harvest at 30-50% amber, it is not recommended to go beyond 75% amber.
Though most people go by trichomes, it is not necessarily the best way. The cannabis bud has an actual look to it when it is finished, the calyxes swell, 95% of the hairs change color, and the hairs recede into the calyx.
What to Cut edit
So its time to harvest but what should you cut and how do you go about it. Well the truth is that this is personal preference. Regardless of how you cut and trim your plants you should touch the flowers as little as possible and avoid breaking up the buds. Damaging the delicate trichomes will reduce the potency of the marijuana.
One way is to leave the plants in their pots (more or less intact) and spread out a couple news papers. Toss large sun leaves on newspaper and do small bud trimming over the other.
Start with the sun leaves and then trim progressively smaller. This way you will have less in your way. For now, leave the buds on your stems. For trimming the buds, first take all the tiny popcorn lower buds and put them where you are going to put your trim these won't be much of anything when dry. Next trim the actual bud leaves, this trim will have lots of trichomes so should be saved to make oil, butter, or something else so they don't go to waste. To trim the bud leaves start with leaves that you can see a stem on and clip off at the stem to remove. Then begin to trim in a circular motion around the bud until you have cut back to the sugar coating. Trimming past this point is just wasteful. Don't worry if they look fluffy or leafy, they will dry and cure into sticky potent buds if you've done everything right.
This should leave you with all the buds on a nice thick stem network that is suitable for any drying/curing process you want to use. Some drying processes will involve just clipping at the main stem and hanging upside down to dry, others will require you to further break down the plant into smaller bud pieces. When you do this leave enough stem for you to handle and work with the flowers you can and likely will trim down the stem later and dried stem doesn't weight a significant amount compared with dried bud. Also, you will find that you can trim the large top buds into smaller buds one node at a time. It is better to leave as large of a single bud mass as possible.