Everyone's dream, and for so few a reality. It doesn't have to be that hard. A flat firm stomach, or rock hard abs if you so desire, are achievable. Yes, that's what I said, they are achievable!
However, if you have been doing sit ups continually for months and are wondering why you have not achieved your desired 6 pack, then listen up.
First things first, if you have excess abdominal fat, it is not only a serious health risk (think high cholesterol, high blood pressure, increased risk of heart disease) but also, there is no way any amount of crunches is going to give you a flat stomach. You need to reduce the excess fat around your mid section first!
So how do you do this? The best way is through a combination of healthy eating (I hate the word diet) and exercise. You need a program specifically designed with this in mind - not one that is designed to develop a six pack on an already stick thin individual.
Your exercise program should include a combination of resistance training and cardio / aerobic exercise.
Weight training will help increase your lean muscle tissue, tone your muscles, and has the benefit of increasing your metabolism for hours after you've stopped exercising. However, don't focus all of your attention on your mid section or abs! Certainly, that can be one of the areas of focus, but in order to have a strong healthy body you need to have a strong core, back, arms, shoulders etc. This means that you should complete a solid 'whole of body' workout that incorporates a regular abdominal component. Your flat stomach will come - don't you worry about that.
Cardio exercise is important as it will help you burn heaps of calories, and will start to get rid of the subcutaneous fat you have around that 'soon to be flat' stomach of yours.
Try cardio exercise at least 3 times a week in addition to your resistance training. You will be surprised at how quickly your flat stomach starts to develop!