For really good Abdominals (abs) diet is key. All the exercise in the world won't bring them out if they are covered by water and/or fat. Furthermore, having good abdominals and gaining weight are goals on opposite ends of the spectrum. You can't really have both.
Most of the models in the pictures (bodybuilders and abdominal advertisements) you see have been under strict diets for 5 to 8 weeks or more. Most also limit water intake for days to further bring out their muscularity for the contest or photo shoot. I would not recommend either if gaining wait is also your objective.
I'd guess that bodybuilders are only at that peak of leanness for about 3 weeks all year. The idea I'm trying to get across is that it's a compromise must be made when considering how much you want your abs to show.
With that having been said, here are some exercises: Keep your abs tight and have a slight forward bend in your stomach area the whole time of the exercise. Don't fully relax at any time. Avoid using weight with abs, as this is known to thicken your waist.
1) Crunches - Lay on your back with your calves on a bench. Put your hands on ears and bring your chin towards the ceiling (not your feet). Your arms and hands shouldn't be used to propel you towards the ceiling. Use only your abs.
2) Leg raises - Lay flat on your back on the floor with your hands under your buttocks (the small of your back should be flat on the floor). Raise your slightly-bent legs so that the bottom of your shoes are about 2 feet off the ground. Now lower until you feet are about 6 inches off the ground. Use a good pace, keeping your abs flexed the whole time.
3) Leg Lifts - Lay flat on your back on the floor. Legs are straight and the back of your heels are on the floor. Hands may be by your side or above your head grasping something for support. Bring your knees all the way to your chest and flex your abs at the top of this movement. Lower your legs until you are at the starting position. Use a quick pace.
4) Twists - Sit at the end of a bench with your feet supported under something. Lean back slightly until your upper-body is about 45 degrees to the floor (sitting straight is 90 and flat is 0, so half way). Place your palms behind your head and extend your elbows out. Now twist to one side and then the other at a rapid pace. Don't go so far to either side that it hurts. This is ideally done using a decline bench.
5) Leg Lifts to Ceiling - Lay flat on your back on the floor. Now keeping your legs in a slightly bent position, raise them to the ceiling. Your body now makes an "L". Lift your tail bone off the floor. You may need to grasp something above your head for support.
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