If you want that coveted six-pack of well-defined abs, it's going to take some work and visual results aren't guaranteed. By following a combination of good nutrition, cardio training, and abdominal training, you up the odds of achieving that goal, with the added benefits of a healthier, more toned physique overall.
First, consider the "garbage in, garbage out" theory. You just can eat a diet of junk and fast food and expect to look like Mr. or Mrs. Universe. If you eat like crap, your body's going to look like crap. Eat a diet of vegetables, fruit, lean protein and whole grains, and you're more likely to embody a portrait of optimum health. Frequent bouts of cardio workouts will help you burn fat, including that which gathers around the middle. Then, you're one step closer toward the goal. Now, you're ready for abdominal work. Doing hundreds of crunches, experts say, is a waste of time. Instead, go for quality over quantity. Perform two to three sets of the following exercises three times a week: Reverse crunches (lie flat on the floor, lift hips toward your rib cage); bicycle (left knee out, right knee toward chest), and plank (lift yourself onto your toes and forearms, keeping body aligned, hold for up to 90 seconds each set) Perform the moves slowly, and with control and focus to isolate the muscles and give you optimum results. Please consult a trainer at your gym for assistance in performing these moves correctly.
Those are the tools. The rewards of a firmer midsection are improved posture, decreased risk of low back pain, and more power in your cardio workouts. Consider these rewards before obsessing over the visual.
Don't forget to add in compound exercises - squats, press-ups, chins, etc - because the most important thing you can do for bringing out abdominal definition is increase the rate at which you burn fat. And nothing achieves this better than big, intense, compound exercises.