Hi Jjangel-
Depends on your goals... If you are trying to build thickness/size in the abs, then train them with weight resistance as you would any other muscle group (brief, intense, infrequently) and eat sufficient cals for growth.
OTOH, if you are looking for more endurance through your midsection (great if you are an active athlete) and are not concerned with muscular size, train at a
higher volume and frequency, and in a
higher rep range (50+ reps, 2 or more times/week).
If you are trying to rip up a bit, you'll need to drop your cals below maintenance levels, along with the training.
Again, eat and train according to your goals, and most importantly, stick to the plan until your goal is accomplished.
Good luck!
Tuna