I would recommend getting a postpartum exercise video (I used Denise Austin and Kathy Smith). I know the Denise Austin one shows how to get the abdominal muscles back together again and I think the Kathy Smith one does too).
Basically you are exhaling as you do a crunch coming up and pushing the muscles back together with your hands as you come up. I don't know if that made sense the way I said it, so thats why I recommend the videos as a safe and sure way of doing it properly. Probably if you "google" on "abdominal diastasis" and "postpartum exercises" you will find a better explanation
Then once they have healed, try "Pilates". There are lots of books, classes and videos on these, they are popular right now. Regular sit-ups didn't help my post c-section tummy get flat, but the Pilates gets them as flat as they were before you had a baby! Take your time and you will be happy with the results