I had it with DS and I know I have it again with DC2. I think they did reattach in between for me. I didn't do anything special though I did do yoga when I could and general physical activity.
I believe the current wisdom for core strength in general is do NOT do sit-ups / crunches since that is an abnormal movement for your body and tends to cause other problems. Things that will help with core strength is carrying your baby, doing yoga or pilates, planks, and non-symmetric weight lifting (where your legs are offset or you do one arm at a time, etc) since you have to use your core to stabilize yourself. Of course these are not specific to healing diastasis rectii. There is an exercise video out there that is specific but I don't know enough about it recommend for or against it. And I can't remember what it's called off the top of my head.