Hi R.:
I'm a certified Pilates instructor who has also been certified in general aerobics and weight training. I've worked with dozens of women over the last five years who have had the same problem and have tried everything (myself included!) Pilates is, by far, the best thing you can do to tone up your abs. If you can spare the extra cash, I highly, highly recommend that you go to a Pilates studio and get some personal training. You can then take the techniques you learn in Pilates (breathing, scooping, etc.) and apply them to any workout to get more out of it.
You can do sit ups till you're blue in the face and they will have very little effect compared to if you were to do them using Pilates methods. If you can't get to a studio, the next best thing is to rent some Pilates videos. For maximum results, I always tell my students to combine an aerobic program (running, etc.) with Pilates to both tone and take inches off. I assume you've cleared exercise with your doctor already? Women with abdominal muscle separation need to be extra cautious and are somewhat limited in what they can do right after they have their babies...so if you have that issue now or have had it in the past, be sure to talk to your doc about any exercise regimen, especially Pilates, before you begin!