When stretching and strengthening any muscle in the body, remember these important tips:
Don't bounce in and out of a stretch. Always hold a stretch statically.
Hold a stretch for 30 seconds. According to research, this is the magic number for muscle stretching. Thirty seconds is the optimal time it takes to produce the best possible lengthening of a muscle.
Always keep in mind where on the body you should be feeling the stretch (i.e. front of thigh or back of the thigh). If you are not feeling the stretch in the location of the appropriate muscle, you probably need to change your positioning.