The piriformis stretch is for piriformis syndrome, sciatica, low back pain, deep buttock pain, sacroiliac joint pain and restless leg syndrome.
1) This Active Isolated Stretch (AIS) is absolutely the most effective: Stretch the gluteus maximus first by lying on your back and bringing your right knee to your left shoulder. To do this you will be contracting groin muscles and stretching the large gluteus maximus that overlies the little piriformis. This position gives a mild stretch to the piriforimis. Actively stretch and exhale as you assist for two seconds with your left hand pulling your knee closer to your shoulder. Release your leg to the floor to allow the blood to flow and repeat 10 times. Stretch the opposite side, bringing the left knee to the right shoulder for ten two-second stretches.
Now be specific to the piriformis: Hold your right knee at your left shoulder as above except with the right hand, then with the left hand, grasp your ankle. Exhale and pull your foot toward the floor. Hold for two seconds, release the rotation and repeat 10 times. Increase the intensity of the stretch by opening the angle at the knee to 90 degrees. Be sure your knee points at the opposite shoulder. If your groin feels tight or painful, stretch the short adductors by bringing your knee out to the side to open the angle at the groin or alternate back and forth between stretching and contracting the groin muscles.
2) Travell and Simons in Myofascial Pain and Dysfunction: The Trigger Point Manual Vol II suggest this stretch: Flex the right thigh at the hip at a 90 degree angle and bring the right thigh across the left to place your right foot on the floor to the left of your left knee. Hold your right hip down with your right hand and stretch your thigh across and down with the left hand, pulling your two hands away from each other.
3) Another way, although less specific, is to bring your leg into a tabletop position flexed at right angles both at your hip and knee . Push out on your right knee with your right hand as you pull your ankle toward you with your left. You may need to use a rope or belt to assist. Use the same Active Isolated Stretching protocol of actively stretching as you assist for two seconds. Release to the starting position and repeat 10 times. This stretch also treats the gluteus minimus and medius of the hip whose anterior fibers medially rotate the thigh.
4) Physical Therapists suggest a similar exercise but it is less effective than the first AIS stretch given above: Place your right ankle on your left knee. Hold the left leg behind the thigh and bring it toward you to stretch the right hip.
5) Pigeon pose in yoga is basically the same stretch: Sit on the floor with your right leg bent in front of you and your left leg extended behind you. Place your forward leg so your shin is perpendicular to the line of your body with a right angle bend at the knee. Bend forward at the hip to bring your torso over your forward leg.