Your abdominals, better known as your stomach muscles, can not only make you look in shape when you're not but provide the stabilization and power for virtually every type of exercise and sporting ability. They transfer force from the upper body to the lower, and the rectus abdominis - the  washboard when finely honed - is the prime mover of your spinal column. The stronger your abdominals, the less likely you are to have back problems.

Abs should be worked in every session.

But forget your old P.E. classes and bag the sit-up, which has long since vanished from the repertoire of exercise physiologists. It's been replaced by the crunch, which provides a better abdominal workout with less stress on your hips and back. (Ignore the inclined sit-up boards as well: Even if you hook your knees over the end, which is how most boards are now built, you're pulling with your calves and hamstrings, which are doing work your abs should be doing.)