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.
Did all that holiday snacking leave you stretching the seams on your Santa suit? Coach Chris Bathke has what you need to get back into fighting shape, fast.