#1 and most important, if you can BREASTFEED! You might not see the weight fall off immediately but it will come off! In fact, any "baby weight" that I have right now if from after I stopped nursing.
#2 start walking with your baby every day as soon as you can. Before I was pregnant I always thought that walking wasn't exercise, but let me tell you, it works! Especially because you are getting a great weight workout at the same time (stroller + baby = 20 - 30 lbs and that is just when they are little!). The other side benefit to this one is that it keeps you sane you have to get out of the house. Even if your baby was born in the winter, bundle the little guy up and get out there! The sunlight is really good for their tiny kidneys!
#3 as they get older don't be afraid to dance along to the Wiggles or sign up for Kindermusik! This get's you moving around with your baby and they get to enjoy the music!
Now of course there is always "eat right", etc. but I have to say, I'm not the person to comment on that... if there is chocolate in the house... well, let's just say it doesn't last long!