Piraformis Stretches using a lacrosse ball: clickherefor the video. Please Note:Eventually this will feel so wonderful that you will want to "sit on your ball" often - please keep it down to, no more than, 5 minutes per day per side so that you do not bruise your piraformis or, even worse, irritate your sciatic nerve!
For general low back pain, the Yoga position called Child's Pose works very well to stretch your QL (Quadratus Lumborm) muscle as well as your Errector Spinae muscles. I recommend holding this pose for 5 minutes at a time. It is relaxing and therapeutic!