Now; post exercise stretching is very important, and BarracksSi has pretty much hit the nail squarely on the head.
Muscles respond much more favorably to stretches held for up to a minute rather than trying to bounce them around and force them to stretch. Another good way to gauge your stretching is to reach the point of mild pain, then back off just a hair and take four very deep breaths. Hold the last breath for five to ten seconds then exhale slowly. Deep breathing facilitates relaxation of the muscles -- the end goal of stretching. Those four deep breaths and holding the last should take from 25 to 35 seconds. Do that twice on each muscle you stretch.