The usual "harvest" rule of thumb is as follows:
A lot of people like to harvest when 75% of the pistils (white hairs) have turned brown. However, this isn't always the most accurate of methods.
A better method is to get one of those small hand-magnifiers from Radio-Shack (or wherever..) so you can see the trichomes (the "crystals" on the buds...) clearly.
When you look at a bud up close, it will have little 'stalk' looking things on it, with round 'heads' on top of the stalks. (I'm sure there's a picture in the image gallery somewhere...) Most folks I know like to harvest when most of the heads have turned cloudy.
If you harvest when the heads are clear, you will have a more cerebral buzz to the high, but obviously your yield will be diminished. You can wait until you see amber heads as well.
Amber heads mean some of the THC is starting to degrade/break down and the high will be a very STONY, couch-lock stone.
Obviously the type of high is influenced more by the GENETICS of your particular plant, than it is by changing when you harvest. But, it can give your knockout indica strain that extra 'boost' if you harvest a little later.
Just a little something there fore you to "smoke on." (And by "smoke on" i mean "think about.")
jE