The piriformis muscle stretch.
The piriformis muscle is situated deep in your buttock and can be responsible for contributing to low back pain, sciatica or general tesnion in your low back region.
The below exercises are simple ways to keep the piriformis muscle from carring tension and keep your low back and buttock area relaxed and strong. The first video below is the basic way of stretching the muscle.
The second video below a more advanced way and requires good knee health and hip flexibility.