Train your abs like any other muscle group, with weights, about 12-15 reps and 4 sets per exercise.
The difference with abs is they are designed to withstand plenty of punishment, so do not NECESSARILY need as much rest as other muscle groups. Unless performing a circuit, take a minute's rest between exercises.
Be sure to taret UPPER, LOWER and OBLIQUE abs. A good structure to follow is:
Day 1: Upper/Lower Abs
Day 2: Lower/Oblique Abs
Rest for one day, then repeat.