Whenever I get on my mat, at home or while I am waiting for a class to start, I like to do pelvic tilts. I haven't even realized that my back was hurting until I begin to work out with these subtle spinal movements. It usually just takes a few minutes of these to alleviate minor back pain and stiffness.
Lie on your back with the knees bent and the soles of the feet on the floor. This is your neutral position, with the natural curve of the lumbar spine causing the low back to be slightly elevated from the floor. On an exhale, gently rock your hips towards your face. Your butt will not actually leave the floor, but you will feel your low back press into the floor. You are essentially taking the curve out of the low back. Repeat this movement 5 to 10 times.