I leave feedback at the conclusion of every transaction. So far, they've all been textbook clean transactions, so feedback has been easy. If, say, a week after a textbook clean transacton has finished, I still have no feedback from it, I PM the other party and ask if they would leave me feedback, or if there was something about the transaction that was not apparent to me which causes them to not want to leave feedback. The response I get in such cases is, "I've been so busy it just slipped my mind." And then they leave me feedback. A friendly reminder that their feedback is valued by me and a little patience is, so far, all it has taken to get the feedback I seek.
FWIW, I think there's room on Head-Fi for two Ken's with a Gecko for an avatar.