I'm a big advocate for stretching after you workout or run. Stretching beforehand can lead to injuries. It is not smart to lengthen your muscles before you demand so much of them. The reaction time of the muscles is slowed by doing this and the joints may be a little less stable.
Check these sites for more info on stretching:
Good stretches to do post-run:
Hip Flexor Stretch
These are the typical 5 stretches that I go through. I do 2 or 3 for each side and hold each for about 30 seconds. I always make sure to come out of my stretches nice and slow so that I maintain what I gained as well.
Do you stretch?