I've heard wearing a gurdle, designed to help your abdominal muscles after pregnancy, is really good for helping your stomach return to it's pre-state. It will help if this is teamed with healthy eating, breastfeeding and excersise.
Nursing: Not only is your milk best for baby, but nursing will help get you back in shape more quickly.
Get a move on: No matter how exhausted you are as a new mom, exercise is still important. It will help build up your stamina and return the muscle tone you lost during the pregnancy.