Sit-ups are a good exercise but there's more you can do. You can do crunches if you want to hit the top abs, or lay on your back and lift your legs up which will hit your bottem abs. If you are really feeling froggy you can get a pull-up bar and do straight leg raises from there. It's killer, you'd have to work up to it. You can also work your obliques (sides) by twisting when you do situps, or by doing side bends.

You can also make your situps harder by using an incline bench, and by contracting your abs harder and the whole time that you're doing them. It's crazy how much the abs have to do with. If you cough, sneeze, poop, or punch your abs are always involved. They also protect the spine.