I use a balance ball. It's not too hard, doesn't put as much strain on your back, but so effective. There are a few different things you can do, like regular crunches, lying flat on the floor lifting the ball with your feet. And a new one I learned not too long ago, in the pushup position, feet on the ball, sliding the ball forward contract the belly and sliding the ball back relax the belly. The back needs to stay straight when you relax the belly, no arching.
I started doing mine 3 month after I had my son. A couple month into it my abs looked better than they were before I had my baby.
You can find more exersizes here
http://exercise.about.com/cs/abs/l/bl_core.htm