PILATES is absolutely the best thing you can do for a bad back. You will strengthen your abdominals (and back) muscles properly without hurting yourself. While you are strengthening, you will also get the flexibility that most people lack, which can lead to injury.
The best thing to do is take one-on-one Pilates lessons, which unfortunately, is expensive, but you will learn how to move properly. However, it's very important to make sure they are trained properly through the authentic method of Pilates. Many so called instructors, have actually been trained with only a few weekend seminars! Find a properly trained Pilates instructor at
www.romanaspilates.com
A lot of studios have an Introductor Offer. Try it out, I guarantee you will feel better after just a few sessions.