There are two keys to defined and strong abdominal muscles.
1) Low body fat allows for the musculature to make itself visible. This can be achieved by a combination of weight lifting (which increases resting metabolism) and cardiovascular training such as running, elipitcal, etc. Of course, consuming less calories is the other side of this equation as well.
2) Increasing the size of the muscles themselves is also helpful as well as independently healthy---abs are a key "core strength" area that support your entire body in all its movements. For this I like to do a combination of things:
- a) crunches where you lift your head and upper body up while also raising your legs, so that both ends of your body can "meet" in one smooth simultaneous motion (many gyms have this as a machine with weights)
- b) leg raises all the way up to my head when hanging by my arms from a pullup bar
- c) SPRINTS!! This is VERY underrated: if you do a series of sprints (100 meter, 200 meter eg) you will notice GREAT ab results. Go to a track, run a 100 meter dash as hard as possible. Walk for 200 meters. (wash =)) and repeat like 6-8 times and you will feel a great ab soreness the next day.