Depending on how limber you are, I have found that pilates have been helpful. Also, going to the gym and doing reverse situps has helped me. (leaning over a padded bar with you feet hooked under another bar, and lifting yourself up) Not sure that is a good explanation, but the best I could do. If you are a swimmer, dolphin kick on your back really strengthens your stomach and your back. Go easy though, when I first started doing this, I gave myself muscle spasms in my back. The dolphin kick doesn't require getting down on the floor, and I think it is easier than sit-ups. Old fashioned sit ups are good also.