The Abs are a short range, very powerful, muscle group. And should be trained like any other muscle group. 2 to 3 sets of 8 to 12 reps should do it. Even 3 to 5 reps for some strongmen training. And 2 to 3 times a week. Anything else is just more wasted gym time. Add weight to most any ab exercise and you will soon seen those deep lumps of muscle appearing.
My view only, but the best investment for ab training is an Ab wheel. At around $4.95 at Wal-Mart, Target, etc. A great bang for your buck. Hit's the lower ab wall, at well as the upper, strongly. Or try doing a regular situp (feet strapped down and knees bent) holding a BB over head. Probaly want to raise to a top crunch position, but a great ab exercise never the less. On most any ab exercise find a way to add weight to the movement. good Luck.