Tighten Up. For flat abdomen eat healthfully ; be physically active. Calories burned must > calories consumed. To tighten ab's: crunches, reverse crunches, exercise ball crunches, pelvic thrusts, lying leg raises, side bridge ; “bicycling” exercise on your back. Engage in regular aerobic exercise (i.e., running, swimming, nordic track, bicycling, kickboxing, walking, cross country skiing, hiking, etc.).
Exercise. Regular exercise including situps will help. If you are able to lose enough weight to get back to your prepregnancy weight, you will likely have lost most of the belly fat.