My friend, Moses, posted a link to this New York Times article in the comments of my last Stretch Reflex post. It has really good information about the myth of stretching before exercise. To sum it up, static stretching (holding a stretch for 30 seconds or longer) temporarily weakens muscle tissue which is not what you want before a good workout. The article suggested it could impair performance by up to 30%!
Currently, the new ideas behind stretching are to perform dynamic stretches before exercise. These are active stretches that prepare you for the type of activity you are about to perform. This could include windmilling your arms, kicking your legs out like a soldier, gentle squats, torso twists or ankle circles. The article gives some more complex examples with diagrams.
Save static stretching for after your workout. This is when your muscles are relaxed, warm, and most receptive to elongation that can lead to better range of motion.
I have learned in my recent studies just how important warm-up and cool-down are for the body. It takes a little extra time but is so worth it to avoid injury, promote faster recovery, and enhance your performance.