I used this ab workout routine in 2012 the last time I won the daily new golden in the theater at Madison square garden. I had a strong core that year.

This workout routine can be done in or out the gym. Just follow the routine. To achieve best results. All are achieved 30-45 seconds rest in between sets

Decline sit-ups 4 sets x 15 reps
Seated knee raises 4 sets x 15 reps
Lying knee raises 4 sets x 15 reps
Knee-reach crunches 4 sets x 15 reps
Crunches 4 sets x 15 reps
Ab wheel 4 sets x 15 reps
Decline alternate side sit-ups 4 sets x 15 reps
Side raises 4 sets x 15 reps
Side leg reach raises 4 sets x 15 reps