The best most efficiant exercise for developing the strength and size of the muscle are AB crunches.
The trick is to maintain constant tension, firstly lying on the floor with you legs resting on a bench enables you to extend and contract without placing any extra stress on the lower back.
Second, hold onto a dumbbell as close to your throat as possible, which increases the moment arm.
Thirdly, squeeze at the top but most importantly don't lose tension at the bottom, don't let you abs relax, this happens when you shoulder contacts the floor.
Maintain a constant rep cadence of one set to twenty.
If you can do twenty comfortably, increase the weight of the dumbbell and do as many as you can, your target is twenty.
Add this exercise to the end of your workout.
After three months of strict repitition and progression, you will notice the size and development of your abs.