Have you tried incorporating jumping rope into your workouts? It gives you an AB-mazing cardio workout while chiseling away at your core with every jump. This weekend, I wanted to get some cardio in without running (it was a rest day) so I brought my jump rope to the gym and incorporated abs between sets to create this workout. It kicked my butt!! You try too and sweat your way to flat abs!
One note: This workout gives you 15 seconds rest between exercises. For an added challenge, only rest every 5-10 minutes.