For those of you who have been training for a while, have a very strong mind/body connection (know how to control your abdominal support with your brain), and are tired of the ab exercises you're seeing over and over again in the magazines, try the following: Begin in  a high plank position (on hands, not elbows), with hands on a medicine ball.  The softer ones are less challenging because they're more stable, whereas the rubber, round hard ones create more core recruitment.  Have your feet on a bosu.

Keeping your abs very tightly pinned up toward the spine, slowly step left foot to left side on the ground, beside the bosu. Follow with the right foot.  Both feet are now on the ground.  The medicine ball will move as it will.  Slowly step right foot back onto the bosu, follow with the left.  Step right foot to the floor, right of the bosu.  Follow with the left. Continue, keeping the abs combatting the instability of both bosu and medicine ball.

Every muscle in your abdominal group (as well as your erector spinae) will be utilized, closely contracted to protect the lower back against the unstable movement. The obliques experience the unusual challenge of the legs stepping to difference levels, switching between stable and unstable ground, while in a plank.

It is a good idea to try this with a partner / spotter when attempting for the first time.

Let me know how you make out!