I would suggest crunches over sit ups, and vary them. Set of 100 with legs straight on ground, 100 with knees bent feet on ground, 100 with knees bent feet in air, 100 with legs straight feet in air, 100 with body curled to the left, 100 with body curled to the right, those will work just about every ab muscle you have. Also, for the lower abs, try dolphin kicks, flutters, and 6 inches. Hope that helps
