Another vote for breastfeeding. Seems to work pretty well in my case, anyway. With both pregnancies, I ended up gaining 35-40lbs....not sure how that happened this last time around, as I was actually losing weight in the beginning from hardly being able to eat thanks to feeling sick all day, and I've been eating a healthier diet than I did with my first pregnancy. Then again, I've read stuff about some women just having a certain amount of weight that they're just going to gain regardless, so who knows. Anyway, I'm two weeks post partum and I've already lost 30 pounds. I lost all of the weight from my first pregnancy within 3-4 weeks, too.
Something to keep in mind is that you're going to lose a lot of weight in the beginning regardless of what you do, simply from giving birth period. The hard part is toning back up, especially if you've had a c-section.
What I'm looking forward to getting back to is the workout video I purchased shortly before I found out I was pregnant with Maddy. I only got a chance to do it a few times before I found out I was pregnant, but those few times were kicking my butt so I know it'll be good for me when I'm able to start it up again.
Here's the info on it if anyone's interested:
Amazon.com: The Firm Stability Ball 55cm: Sports & Outdoors
One note I'd make is that I got this sized ball based on their recommendations for height, but I think I actually would have preferred the next size up. I'm between 5'3" and 5'4".