Check the growing forums.

In short though, you have to inspect the trichs on your buds with a magnifier to know when to cut. You can buy them at The Source (Radio Shack) for $10. You need at least a 30x zoom scope to get a good look.

Harvest when trichs are 100% cloudy, even a little bit amber colored.

If the trichs are clear, that is too early and will result in loss of potency, and yeild.

If you wait too late, the trichs start to turn amber/red, then that is a sign that THC is beginning to break down.

I like to wait until trichs are 80% cloudy, 20% amber.