The trichomes are the crystals that form on the plant. They are shaped like a golf ball on a tee if you look at them under a microscope. They are the best thing to judge the ripeness of a plant by. If they are all clear, it is immature. If they are mostly cloudy, it will be a heady, up kinda high, if they are mostly amber, it will be a couch-lock kinda stone. Most people harvest when they are all cloudy with some turning amber.

As long as the trichomes are clear, they are gaining potency, cloudy = peak potency, amber = losing potency. Pick up a 10 dollar microscope from radio shack, it will help a lot.

If you don't want to go by trichomes, you can make a kinda educated guess about ripeness by the hairs. If all the hairs have turned reddish/brown, its probably ripe. This isn't the best way to judge though, because some strains are completely ripe before the hairs change and some aren't ripe til after they all change.

Here are a few good links:
http://www.drugs-forum.com/growfaq/1529.htm
http://boards.cannabis.com/showthread.php?p=846276