Tennis balls can make great self massage tools.
The simplest method is to stand with your back to a wall, trap a tennis ball between the wall and your body, and then just wiggle around. This works great for your lower-back, but can be much harder to do if you have upper back pain because the tennis ball will tend to pop out and fall to the floor. This is easy to get around - take a thin sock (the thinner the better), and stick a tennis ball inside. Now you have a handle which you can use to pull the tennis ball around, which will prevent it from popping out, most of the time. I've even been able to massage my neck using this technique, something you cannot do with a plain tennis ball.
You can also give your hips or lower back a great deep-tissue massage by trapping the ball between you and the floor, and using your body weight to supply the pressure. I find, however, that this can be too much pressure. To spread the weight out a bit, I use 3 tennis balls, stuck in a row inside a sock, with the sock pulled as tight as possible and tied off. This gives you a relatively hard tube/roll that you can place under you, and roll around on. You'll need a relatively tough sock that isn't too stretchy.