Try bellydancing. May sound a little strange as a form of exercise, but it really works out your body and will tone your abs really well! (As well as your legs, your arms, your hips, your bum, etc). Look around in your area...a lot of belly dance troupes offer classes. If not, there are several good tapes/dvds out there.
That combined with a healthy diet and perhaps some aerobics or walking should take care of things.
I've heard good things about pilates, and I've tried it a few times, but I've not had the time to do it regularly. It really is supposed to help strengthen and tone your muscles. My boyfriend's sister is a dancer and swears by it! And she has a great figure!
If the stars are aligned against you, realign the stars.