How to Strengthen Your Stomach Abs
The two main factors affecting the shape and appearance of your stomach abs are muscle mass and fat. However what many people don't realize is that there are several abdominal muscles, and simply doing crunches/sit-ups are far from what's required for six-pack stomach abs.
The following are some rock-solid principles of acquiring strong, good-looking stomach abs (and the abdominal area in general).
Reduce your body-fat percentage. This means doing regular cardio exercises to burn fat, weight-training to increase muscle mass for a greater resting metabolic rate, consuming less calories than you use each day, dividing your daily food intake into several smaller meals (to prevent slowing of metabolism and fat-storing insulin spikes) and restricting your intakes of refined carbohydrates (like white rice, white bread and potato chips). The aforementioned points are almost canonical in nature regarding their acceptance in the diet and fitness community. Reduce your body-fat, and the goods beneath (your muscles) will show.
There are plenty of skinny people with six-packs who just look sickly and weak. To actually strengthen and build your stomach abs, you have to exercise them to make those muscles grow. As with any other part of your body, exercising abdominal muscles with high-intensity exercise is the key to making those six-packs bulge out. Doing 1000 light crunches a day starts to become pointless. Instead, start with 50-100 sit-ups a day, and once you can do that easily, do the same number of sit-ups while hugging a 5-10kg dumbbell to your chest. Keep using heavier weights until you are satisfied with the muscle mass of your stomach abs.
Back in college a slightly chubby friend of mine started doing crunches every day to get a six-pack stomach. The result? After a couple of months his belly had a firm bulge in the middle. There was no six-pack in sight (except for the six-pack of beers in his room). Not only had he not reduced his body-fat, he had only worked out the stomach abs on the front resulting in an abdomen that looked like that of a pregnant woman. There are abdominal muscles on the sides above the waist called obliques, and working on them is extremely important to sculpting the shape of your stomach abs. When doing sit-ups, do lateral/side ones where you alternately turn your upper body left and right as you do the sit-ups. This will work out your obliques and improve the appearance of your stomach abs even before you lose body-fat by improving overall structure of your abdominal area.