You need to change your routine around often, as muscle adjusts to any constantly repetitive motion and your gains will slow and plateau in time. My experience is that stomach muscles do this less so than other muscle areas, but you will hit a wall sooner or later...
I usually do a variation of 8-10 different excercises on my stomach every other day, which lasts about 10 minutes. High reps are a waste in my opinion; the various crunch routines I do only see up to 30-40 reps with everything else seeing 15-20. The key is variety; you want to shock your muscles.
High reps will get you there, but I get MUCH better definition out of juggling a large variety of excercises with a low to medium number of reps than plugging away at 3-4 excercises and a ton of reps. Even shuffle the areas you start on first (lower, mid, upper, side). I've been told to start with lower abs and work my way up, but I meander all over the place...
Get a book on ab excercises and play around with them. It will take a while, but you'll find a good mix of excercises that will give you a hell of a "burn" and work best with your body. You can go broke buying ab isolation equipment; a bench, a gym mat and few medicine balls are all you will ever need if you adjust your routine around.
Finally, fat is burned off in layers on your body. Anybody telling you that they can target fat burn is selling something or they don't know what they are talking about.
By the way, I'm 30 with a 32 inch waist at 165 lbs. and I've carried an 8-pack since I was in high-school, so I'm obviously doing something right. A lot of it is the type of diet I keep and my genetics, but I won't totally dismiss my routines, either.
