I do tri-sets for abs like this

first set
1) upper abs - crunches
2) obliques - twisties (legs and upper body 45 degrees off of the ground twist the torso while holding a medicene ball)
3) lower abs - leg raises

second set
1) twisties
2) leg raises
3) crunches

third set
1) leg raises
2) crunches
3) twisties

To answer your question for strong abs you really should go to failure. On these set's I start with 20 for each and on the last set I go to failure.