Cold muscles are more likely to tear than those that have had a chance to warm up. Stretches should be slow, precise and deliberate. A stretch that takes 30 seconds to execute is far superior to a three-second quickie. It is better to stretch for five minutes and do three or four good stretches than to try to turn your session into an aerobic workout. Stretch a muscle until it becomes slightly uncomfortable, hold it for 10 or 15 seconds, and slowly release.
Body position is very important.
Keep your back straight. If possible, stretch in front of a mirror so you can keep an eye on your form. Concentrate on keeping a straight back, firm belly and tucked-in hips. Don't bounce, but rather exert a steady effort. This is also good practice for control in climbing.