I adapted this abdominal exercise from one that I learned in a great group fitness class at Equinox. You need to be seated on a hardwood floor or other surface that is slightly slippery (linoleum is fine too).
Begin seated on the floor, knees bent, feet flat.
Place a folded towel beneath your heels.
(Your palms just barely graze your outer ankles).
Simultaneously, slide your heels forward (straightening your legs) as you lean back (until you are almost lying flat on your back).
Then, drag your heels back in toward your bum, as you sit back up.
Repeat. 10 times. 1-3 sets. Be sure to go slowly. [Results look best when paired with a bikini].