First of all, CONGRATULATIONS on your new beautiful little one. Secondly, if you decided to breastfeed, not only will you be giving the best for your baby girl, but you are also helping your body get back to normal as well. After my second baby I decided that I was going to lose all the weight I had gained with my first baby. I read a lot and learned that calorie in/calorie out doesn't matter as much as carbs. If you don't process carbs, your body can't store fat. Change your diet and omit all the starchy foods and anything that breaks down as sugar. Stay away from ALL sugar (natural sugar from fruits and vegetables don't count so eat a lot of those). You want to limit your carb intake to less than 100g/day. I did that and lost 75 pounds and I have kept it off. Don't cook with vegetable oils and such because they have trans fats. Saturated fats are ok so eat lots of good meat like steak, hamburger, chicken and turkey. I have changed my diet and continue to eat like this and I now weigh 128 lbs and I'm down from 215. If you want an authority on this type of eating, check out Gary Taubes (spelling?). It is radically different than what most people say, but it has worked for me.
As far as the Slim Fast drinks, they are full of sugar and will make sure that you store fat.
I hope this helps you and again, congratulations on your little one.