1- Force The Contraction. When performing abdominal exercises you shouldn't just go through the motions. You should concentrate on your mid section creating a strong contraction. The best way to develop your abs is to work them as hard as possible on each rep.
2- Hold The Contraction. Once you've contracted your mid section, lower your body to the mid point of the move and hold the contraction for an extra 2 to 3 seconds. The strong contraction will work the muscle fibers in your abs and teach your body to develop your mid section more effectively.
3- Stretch. After the contraction stage and as you lower your body to the starting position, you should feel a stretch in your mid section. This stretch in the negative phase of the movement forces your abs to work throughout the entire set, taking your abs to full exhaustion with fewer reps.
This will lead to increased intensity and greater results.
4-Quality Over Quantity. If you follow the above tips, you'll be achieving this goal by default. Don't be obsessed with pumping out meaningless reps. Emphasize the quality of each rep, forcing and holding the contraction and feeling the stretch. No need to count reps, just work as hard as you can during each rep until you have to break form, then stop and rest for the next set.