Traditional sit-ups emphasize sitting up rather than merely pulling your sternum down to meet your pelvis. The psoas muscles run from the lower back to the front of the thighs. This muscle action is to pull the thighs closer to the torso.
This action is the major component of sitting up. Because of this, when you are doing sit ups your psoas muscles are the primary muscles being engaged and not your abdominal exercises.
So, sit-ups are a pretty useless exercise if you want to get your abs in shape. Instead, use crunches, because they directly work your abs.
The Right Way
Lie flat on your back with your knees bent and your feet together flat on the floor and about 10-15 inches from your buttocks. Your hands should either be crossed on your chest, by your side, or cupped behind your ears.
Without moving your lower body, curl your upper torso up and in toward your knees, until your shoulder blades are as high off the ground as you can get them. Only your shoulder blades should lift—not your back. As you come to the highest point tighten and flex your abdominals for a brief second. Slowly lower yourself back to the starting position. Repeat for reps.
Remember crunches only tone the ab muscles, you need a lowered calorie intake to burn off the fat, so the muscles then show through clearly.