One of the best yoga poses for stretching the hamstrings, Reclining Big Toe Pose (Supta Padangusthasana) also stretches the hips, groin, and calves. Done properly, it will even strengthen the knees.
How to do it: Lie on your back. As you exhale, bend the left knee and pull it toward your chest. Keep the other leg pressed firmly onto the floor while pushing the right heel away from you. Hold a strap in both hands and loop it around the middle of your left foot. As you inhale, straighten your left leg slowly toward the ceiling. Move your hands up the strap until your arms are straight, while pressing your shoulders into the floor. Once your left leg is straight, engage the left thigh slightly and pull the foot toward your head to increase the stretch. Stay here for 1 to 3 minutes. Then lower the left leg slowly toward the ground, keeping the right thigh pressed into the floor. Continue until the left leg is a few inches off the floor. Work the foot forward until it is in line with your shoulders. Inhale your leg back to vertical. Lower the leg and repeat on the other side.
How to get better: When you extend the leg upwards, press the heel toward the ceiling. Once the leg is straight, engage the thigh slightly and lift up through the ball of the foot.