If your abs look good, your whole body looks good. Having Big arms and chest is one thing but having rock hard defined abs is so another.
Everyone knows that nothing turns heads like a defined full set of abs. And hey, chicks love em. What more needs to be said.
Contrary to popular belief, the abs or Rectus Abdominis is really only one muscle. It stretches from the top of your pelvis up to your ribcage. The "six-pack" as it is called is merely sections of this one muscle.
Dispelling the Ab Myths:
The action of any muscle is simply to contract. And as you can't contract half your bicep or quad, it makes sense that you cannot contract half your abs as well. Therefore the myths about "upper ab" and "lower ab" training are total nonsense. I'm not discounting those training methods but the simple fact is that if you can't see your lower abs, it just means you've got fat covering them, not that you need to work them harder.
Say goodbye to hanging knee-raises, straight-leg raises, and any other type of leg raises for building your abs. They simply don't work. The primary role of your abs is to stabilize your body. When you raise your leg for example, your hip flexors pull on your spine and arch it forward/downward. Your abs stabilize this action by resisting and essentially keeping your back from breaking. Take note that this is the static action of the abs. It doesn't build or strengthen. Leg raising exercises only feel like they are effectively working your abs because of the repeated strain and exhaustion of your muscle producing catabolic, lactic acid build up. Search for "lactic acid" at http://www.muscle-body.com for more information.
What Really Works:
What does strengthen your abs is what strengthens any other muscle in your body. Contraction! In this case - Decreasing the distance between your rib cage and your pelvis.
So we come back to the good old sit-up. But there are a few things to keep in mind.
Firstly, forget about the cool all-the-way-up sit-ups you see in the movies. Your abs are fully contracted when your shoulders are about 5 to 6 inches off the ground. So don't bother trying to impress or outperform someone by touching your elbows to your knees. It does nothing for you. Crunches as they are sometimes called are the way to go.
Secondly, don't wedge your feet under something to help you get up. You don't need to do this if you are doing crunches properly anyway. Your legs should be at 90 degrees to each other and your knees at 45 degrees to your waist. If you need to, use something to press your heels back against to stabilize yourself. This way you will use your hamstrings instead of your hip flexors which will keep the focus on your abs.
Thirdly, keep your hands lightly held against the side of your head or crossed on your chest. Don't clasp them together behind your head or neck. If you do you will be tempted to pull with your hands to get the last few reps out, which will put unnecessary strain on this fragile part of the spine.
Another good exercise for abs is kneeling crunches. You will need special equipment to perform this exercise. This is where you kneel and contract your abs so that your upper body arches forward/downward. You hold a rope attached to a pulley with weights for extra resistance. Most gyms will have a rope attachment to the triceps pull down to perform this exercise. The idea is to lock your arms against your upper body so that you pull down only with your abs and not your arms.
Ab Training Devices:
In 2 words - Forget it.
You don't need them. Most of them do nothing for you. Building muscle is hard and it hurts. Most of these devices are designed to take the pressure off your body making the exercise seem easy and con you into thinking you are really targeting your muscles. Don't believe the hype. Ab training is definitely hard work but definitely worth it's reward.
The difficulty in obtaining great abs is not so much building the muscle. It's cutting enough fat from your body for them to show. You probably know by now that spot-reduction of fat simply doesn't work here. In order to show those abs you need overall fat loss.