The best way to get your body back into shape after having a baby is to exercise!
Here's a quick and simple abdominal exercise shared with me by my sister, a certified fitness instructor:
Using slow, controlled breathing, inhale deeply. Allow your belly to fill up and rise with the air. As you exhale, tighten your abdominal muscles to push the air back out. Repeat with each breath as many times as desired.
This can be done in the bath tub or whenever you have a few moments to focus on breathing. It's also safe to be done immediately after giving birth, as long as you feel up to it. It's simple and can help strengthen your stomach and core.