How to warm up / stretch and cool down?
Stretching has become somewhat controversial among exercise physiologists.   I listened to one such lady on NPR's Science Friday a couple of months ago, and she said that the university she worked out of had conducted a number of fairly extensive tests and surveys indicating that very little benefit accrued from this practice.
Other than that it felt kind of good. Wired's Pre-Exercise Stretching Is Killing Your Workout article discusses such studies.

I've never bothered with pre-ride stretching.  In fact, the standard wisdom is that one should first warm up and THEN stretch.
Ride slowly, let the legs get used to spinning and the arms used to bearing a bit of weight before increasing your pace.
I used to stretch post-ride; the standard quad and hamstrings stretches and also calf-muscle stretches by standing with my toes on a stair and letting body-weight stretch the calf... But I admit I hardly bother anymore.