the absolute best core/ab work out you can do is the Pilates bicycle I described above. Scientific studies it is the single most effect abdominal workout there is. Make sure you exhale as you extend and inhale as you switch legs.

Planks and side planks are also extremely beneficial, because they work the core and the rest of the body too. Start with holding planks 15 seconds at a time -- build up to 2 minutes. As you get stronger hold the planks for longer building up to over a minute per side.