Yup, IMHO tyler has it right. When you see trichomes on the upper leaves and around the buds, the pistils are turning orange and the trichomes have turned opaque or even started to show some amber (my personal preference) then they are ready. I trim and save all my sugar leaf_ not only the leaves that are trimmed from the buds themselves but also any leaves that are covered with at least 50% crystals.
I use this to make dry ice hash which I then carboxylize and use for edibles. Some strains are particularly good for making hash because they consistently produce really dank, sticky, crystal covered buds-including not only the bud leaf but the fan leaves on the top half of the plants too.The Ayahuasca purple I grew last time even had trichomes on the stems! Awesome strain. Also have had White Widow, Trainwreck and Hash Plant produce similar results.