Spot-training might feel like it's making a difference to fatty areas, but while crunches will tighten your tummy, they won't actually do anything for the layer of fat surrounding the muscles. That's because muscle training builds muscle; it doesn't reduce fat. To burn fat, you need to do cardiovascular exercise.
The American Council on Exercise states that doing "100 sit-ups a day for a flabby stomach will increase muscle endurance for the abdomen, but will not burn off the fat in that area."
The best way to shape up your tummy is to do crunches to help tone your body and build muscle strength, and add a power walk or some other cardio activity to reduce the body fat around your stomach and other areas.