This has probably been mentioned by your physiotherapists but in some cases (depending on whether or not your back is ok otherwise, i.e. no pathological probs) strengthening the back muscles together with your abdominals helps.
It helped me when I had problems with mine (due to sports injury). The annoying bit was that despite seeing the best specialists none of them gave me any specific exercises to do. Even the physiotherapists failed to help me. I ended up ordering a book on kinesiology (strength training basically) that showed an exercise that forever cast away the pain.
As I said, it depends on what you got to start with.
Regards,
Cosmo