One of my favorite ways to hide my poochy postpartum belly is to wear tunics. Wearing one with a little give in the lower part of the shirt but a good fit in the sleeves and bodice ensures that you look neat and your tummy can stay disguised. You can find shirts like this at Marshall's and at ethnic stores, but I'd love to find some online sources. I love the ones pictures above from J. Jill, but they are quite pricey... especially for a mama with a baby prone to spitting-up!!
Note: If you area nursing mama, be sure that you can feed your baby in the shirt before you buy!!