The Pilates circle is an excellent tool for stretching the hamstrings.
Lie on your back on a mat or on the floor. Place one foot in the circle, while holding the circle firmly in your hands. Start with your knees slightly bent.
Exhale and straighten your knee until you feel a slight stretch in the back of your thigh. Hold for at least 20 seconds, then switch legs. Repeat 2 or 3 times on each leg.