As with every other goal here, having defined abs is a matter of having enough muscle mass and low enough body fat. Generally the hardest part of getting a six pack is losing enough body fat to make the bottom portions of the abs show well enough. Just remember: you lose 3lbs of fat the exact same way you lose 30lbs.
For some great information about good exercises to build muscle in your abs and core, have a look at the Muscles and Muscle Groups page.