Just passing along something one of my MRC counselors suggested to me to get the core under control. I have yet to prove it, but I imagine it will be incredibly helpful. Sorry I cannot give my personal endorsement, as I have not started to put this to use yet!
She suggested the Bosu Ball (a balance HALF ball). Anything you do, no matter how simple, requires you work to keep your balance, therefore firming in the process.
I purchased mine at Dick's Sporting Goods.
here is a video on youtube to demonstrate:
http://www.youtube.com/watch?v=M8mcHyfFvQI
there are many other demos as well on youtube.
All the best!