more defined abs

To get more defined abs, you must realize that a lot of definition and the "showiness" of abs comes from a slimming down, mixed with ab exercises. Doing crunches DOES NOT make you lose fat, they do however strengthen your core and build the upper region of your abs. This is useless however if you have too much body fat.  If you find yourself to be a skinny guy with hardly any muscle tone, well then you're in luck. if you're skinny you obviously have less body fat, and with many different ab workouts you'll get those toned abs. So if you find yourself slightly overweight, get alot of cardio before attempting to get those washboard abs.
There are many different ab exercises and routines that can help shape your abs into stone. Do not rely solely on a few sets of crunches per day, because you'll be wasting your time. While crunches are good, they don't work on every part of your abs, and do not offer enough stress on the muscle tissue (after a week or so) to actually allow your ab muscles to hypertrophy. So for beginners here's a little routine for you.
-20 crunches (3 sets), mountain climbers (40 seconds) prone plank (as long as possible) side planks (as long as possible) , 2-3 min jog or run.