If you wan't a well-defined abdomen you need to reduce your body fat. Building up your ab muscles won't be noticeable if you have a layer of fat covering them. To lower your body fat percentage, choose lean protein and vegetables as your staples, and good fats (omega 3s and 6s) in moderation. Limit or avoid altogether sugar and simple carbs (including beer!), while doing 30-60 minutes of cardio 3-to-5 times a week. In conjunction to cardio, crunches (front and side crunches) and weight lifting will build lean muscle and speed your metabolism. The more disciplined you are, the faster you will see results.
And be careful about many ab exercises, since many can cause back problems later on. In fact, most crunches work the lower back as hard as they do the abs, and all that twisting can be a strain. Not that anyone knows how to best work the abs, though, as far as I can tell. For six pack abs, quite frankly, most who have them don't get them through exercise -- it's a sure sign of steroid use, since the belly should have some fat. You'll never see agricultural or hunter-gatherer cultures with flat stomachs. Other than this, the above post is right on.
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