A toned flat stomach improves support of the lower back and helps us avoid overeating simply because the pressure of muscles holds the stomach in its correct position.
To do the basic crunch, lie on your back on the floor or on an exercise mat with your hands to the side of your head or crossed on your chest. Keep your knees bent at a 90 degree angle with your feet resting on the floor. Lift your shoulder blades of the ground as if you are trying to touch your knees with your chest but do not move the legs. Concentrate on lifting using your stomach muscles only.
At the top of the movement contract your ab muscles intensely and then slowly lower your body down. Lightly touch the floor with your shoulder blades and repeat. I do a hundred crunches every morning and then hit the pool for laps.