Abdominal crunch is the best exercise if you want to flatten your stomach. But before you proceed you need to make few things clear here. Suppose you are fairly overweight. You have fat all over the body, and also in the abdominal area. Now if you do abdominal exercise, how much help it would be. I saw many people buy ab crunch exercise equipments, to flatten their stomach. Most of them have high percentage of body fat. Now will those equipment work for them? No, they will not. You have to remove first fats from the body. The main function of those equipments are to build muscles, not to remove fats. So they will not be that much useful if you have fats on your stomach. So it not sufficient to do the crunch exercise alone. At the same time you have to be careful about the food you are taking. If you take too much food, and at the same time exercise a lot, still it will not work.