E.M.
Hi K.,
What you have is called a rectus diastasis and is very common after pregnancy. The best way to get rid of it is to do lots of crunches, while also making sure to "hold yourself together". You can do this using your hands, or wearing an abdominal binder, or just wrapping a towel tightly around your midsection while you are doing your ab work.
The other BIG thing to do is to AVOID DIAGONAL CRUNCHES or any other exercise that targets the obliques until your separation is back to normal (2-3 finger widths or less). If you work the obliques, you will actually be pulling the two halves of rectus abdominis farther apart, making the problem worse.
Good luck, and have fun with all those crunches!
-E.