Treat the ab's like any other muscle group with maybe 2 set's of 8-12 rep's at the end of your regular workouts. Add weight when needed because, as a short range of motion muscle, the ab wall is very powerful. No need for marathon ab workouts by anyone. Keeping the ab wall strong insures balance with the lower back, imbalance can produce lower back problems for some.
Those $4.95 Ab wheels you see around are a very good investment, not a toy as some would believe. It can hit the lower and upper ab's very well. I wonder if some of those Pro's belly's are so big that they prevent doing direct ab work like simple crunches, etc. Those deformed and overly large internal organs, pushing outwards, could be a concern for some of them I guess. Good Luck.