Finally, work your abs 2-3 times per week. Work your abs like any other body part. Training them more than once or twice per week is unnecessary. More is not necessarily better. I typically do only two different abdominal exercises.
Contraction is everything when working abs.
Remember that your abs work like an accordion, not like a hinge. So when doing crunches, you should concentrate on crunching together rather than moving your body upwards. This will maximize muscle tension. You must concentrate on contracting your muscles as tight as you can. This will cause them to fatigue faster.