the typical male will start seeing the abs in more detail around the 10-11% bodyfat range..
Lower Abs : leg raises..
Use the sit up bench, but lay on it backwards.. head facing upwards, and hold on to anything that you can get your hands on.. raise your legs in a slow motion to an upright position, and then, lowering your legs back down slowly, in twice the time that it took you to raise your legs.. ( 3 sets of 20 if you are just starting off )
Mid to Upper Abs: your typical crunch
Lay on a mat, legs/knees slightly flexed, arms crossed over your chest, or however you feel comfortable.. Curl your upper body, upwards, as you raise your shoulder blades / back up off of the mat (once this far up, this is as far as you need to raise yourself) As you raise upwards, squeeze and contract your midsection while in the raised position, pause for 1-2 seconds, then lower yourself back to the surface. ( 3 sets of 20 if you are just starting off )
If your metabolism is slow (like mine), you will need to maintain a diet with moderation in carbohydrate intake.
If you have that sweet tooth, you are going to be struggling.
The key to your abs actually "SHOWING", are everything that everyone has mentioned.. The proper diet, abdominal work, and your fat burning cardio plan.
You dont have to kill yourself on the cardio.. 3 - 4 cardio sessions, at 30-40 minutes a week should suffice, even if you follow a small amount of decent nutrition tips.
Actually, too much cardio can be harmful. You'll burn up excess carbs, fat, and protein, if you dont do the proper amount.
Your body begins to go to alternate fuel sources approximately 20 minutes into some moderately heavy cardio activity. Thus, these alternate sources being stored fat.
Of course, this is assuming you are doing your cardio at times OTHER than the morning, when your blood-sugar is low. This is known as one of the best times to do cardio activity.
Oh well.. enough of my two cents..