A few tips on stretching correctly may help you prevent doing more harm than good when it comes to this activity.
Years ago I was taught we should stretch in our warm up process before physical activity. This may hurt our muscles if they are not warmed up enough first. It is a good idea to at least do enough activity to be sweating a little before beginning to stretch. The reason for this is our muscles get stiff from inactivity because blood flow in muscles is greatly reduced. When we use our muscles blood flow in them is greatly increased and they are much more receptive to stretching without being hurt.
Also, when stretching it is good to hold a stretch no more than about 2 seconds. This is because after 2 seconds in one stretch position our muscles begin to instinctively retract to protect themselves as the muscles' oxygen supply begins to run out. By contracting our muscle after 2 seconds we allow oxygen rich blood to again flow into the stretching muscle. Reapplying the same stretch to the same muscle after even a one second break after the contraction, will allow the muscle to stretch further without our protective reflex hindering a stretch longer than 2 seconds.
Our stretch reflex starts to happen in about 2 seconds, before we can feel any pain. Our body knows when a muscle is running out of oxygen before we consciously can feel it. Knowing this can help avoid injury during stretching.
For more info on stretching you can go to www.stretchingusa.com.