You will want to get yourself a mini microscope, Radio Shack sells on for $13, or a jewelers loop. Look for something with at least a 30x magnification. Once you start to notice yellowing leaves and pistils turning red or brown it's time to study your Trichomes. The Trichomes will tell you when to harvest by studying their color and the ratio of color.

The trichomes will start clear, then turn cloudy, then amber. If let go too long the stem of the trichome will also turn amber and then the top of the trichome will burst. The ratio of amber trichomes to cloudy and clear is a personal preference. It is commonly accepted that more amber will produce a heavier stonier feel where the cloudy to clear gives that heady high.

I personally look for a ratio of about a third amber and mostly cloudy for the rest. Try taking off buds at different times. Label them with the trichome ratio of amber to cloudy and see what you like the best.