Use these two exercises as a progression from the ab wheel. The videos don't do them justice. You've got to try them to understand it.
Keep these in a low to moderate rep range, multiple sets of 5-8 reps. Going high rep on them can make the low back kick in. But you've been warned: This will leave even veteran lifters shaking like a leaf on a tree.
If you understand your opponent, you can manipulate him and win the battle. Similarly, if you understand why weight lifting builds muscles, you can control those babies and make them do your bidding. Muscle Master Christian tells you how.