I would like to just add that if you have a healthy diet, swimming or running on a regulary basis help to tone up your abs. You can combine it with 100 sit-ups or belly crunches but you have to make sure you don´t strain your back. You can do leg raises with your hands on the ground to help you keep your low back down. If the goal is to get a flat belly, doing this should give you visible results in about a month. The most important thing is to be healthy and balance everything.