Genetics also plays a role in whether or not your clients can obtain a flat stomach or a “six-pack” look to their abdominals. Having said that, two types of exercise can help: strength training and cardiovascular exercise. The abdominals are just like any other muscle group: For their definition to become visible, they must grow larger and the fat that lies over them must decrease. What makes the definition of the abdominals so difficult to see is that they are situated in the area of the body that contains the most fat. Strength training the abdominals is only half the story. Your clients will get a flat stomach only if they combine strength training with cardiovascular exercise to get rid of the fat. Most clients do not do nearly enough cardiovascular exercise to decrease their body fat percentage to a point where they would see their abdominals. Even when the aerobic exercise stimulus is adequate, the role of diet must not be underestimated. All people with a flat stomach or six-pack have a very low percentage of body fat.
Abdominal crunches are just as effective as any piece of equipment to train the rectus abdominis muscle, the main muscle in the abdominal region (Demont et al. 1999; Vaz et al. 1999). As your clients improve their abdominal strength, they can make crunches more demanding by performing them on a movable surface, such as a resistance ball (Vera-Garcia et al. 2000). Source: http://www.ideafit.com/fitness-library/top-10-most-frequently-asked-questions-in-a-fitness-center-and-their-answers