Abdominal Routine
My abdominal routine consists of three sets of crunches. Lie flat on the carpet in the bedroom and rest your lower legs on top of the bed.
Beginners may wish to start with three sets of 50 crunches (or however many you can do until they burn), resting a couple minutes between each set. After a few months, however, you'll find that your abdominal muscles have adapted to this movement and you may be able to knock out a couple hundred of these during each set. If this occurs, do the crunches more slowly and deliberately.
Muscular definition in the mid-section (or anywhere else in the body) is a function of both bodyfat levels and muscle tone. The average American male is at about 15% bodyfat; 11% percent is considered lean. Male marathon runners frequently hit 8% bodyfat. Unfortunately, definition in the mid-section of men isn't really noticeable until they hit 7% or so. In my photo, I'm at 6.2%.
I don't believe there are any health benefits in going into the single digits. The benefits are entirely cosmetic and there is evidence that going too low (such as 5%) for sustained periods of time can be deleterious to your health.
Women can have decent abdominal definition at 12% bodyfat and should not go into the single digits for protracted periods.