I had a splint that I used - and it doubled as a back brace. When I did ab work, I just used a scarf tied very tightly around my waist.
At 3 weeks pp now, I'm back in my pre-preg clothes, and my stomach is back to flat again.
That's not to say that I don't have a lot of work to do to get back the muscle tone in my rectis abdominis! But I had a 4 finger split in my abs at the end of my pregnancy, and everything is back together and coming along nicely now.
I highly recommend a splint, girdle, brace...something!