Lets get this popular misconception cleared once and for all. Crunches & sit ups alone will not get you the flat stomach you want. You can do hundreds of them and all you will get is bigger ab muscles. And they will remain covered by the fat layer on your stomach. This combination might even make your stomach look bigger.
Don't get me wrong stomach exercises (crunches, leg raises and many more) are a great way to train your stomach muscles and are necessary if you wish to get a sexy looking flat stomach or six pack washboard abs, but they are only one third of what you should do if you wish to have a flat stomach.
To get a flat stomach you need to lose the fat layer that is covering your abs. combine crunches with correct eating habits and cardio exercises.