I'm not a big fan of lengthy static stretching sessions. But, it helps to work out the kinks in certain areas. For a distance runner, I have my athletes pay attention to their IT bands, hamstrings, and achilles. When we stetch, I have them hold a stretch for at least 35-45 seconds so the Golgi tendon organs can be activated in order to produce a good stetch.
Most people confuse stetching with warming up. Warming up means that you get good blood flow to the performing muscles and engage in activity that results in optimal range of motion. Stetching can aid in that somewhat, but after a period of stetching, you would need to warm up again. So sometimes you see distance runners jog, stretch, and then do some gentle accelerations to get warm again. I have been working on implementing the active warm up that Brent McFarlane has athletes do which combines form drills, running, and selected static stretches. I'd say that static stretching at the end of a meet or training session would be better