I was very active during my second pregnancy and lost weight so quickly that my skin didn't retract, and I had that big flap hanging over my c-section scar (actually made it difficult to heal, but I guess that's TMI. Oh well).
Since then I've filled that flap back up with fat, but I know as soon as I get my weight down that bit of belly will always be there (even when I was thin, I always had a little belly). It's the curse of motherhood (offset by many benefits of course) that most women's bellies are ruined I think by pregnancy.
Ab exercises won't make the flab go away, but it will be firmer and toner in the area underneath the flab, so that's got to help some.
Honestly, I never thought I'd go for surgery, but since I have the c-sect scar now anyway, that's my present to myself once I get down to my goal weight -- get a mini tummy tuck and cut that extra skin away. I'll have them cut along the current scar, cut a couple of inches of skin off, pull the skin down and sew it back up. Plan to get my abs in shape first. May or may not need lipo. In a mini tuck, the belly button isn't repositioned and ab muscles are left alone plus recovery time is quicker. Here's a good description of it:
Sorry, you probably weren't looking for such an extreme option.
I do have one of those big rubbery balls you can lay over on your back and use to do various sit ups (and other exercises) with and without weights. An instruction sheet came with it, but didn't have many ab exercises. I plan to look up some ab exercises online. I use it mostly to stretch my back and shoulders out (used to carry much tension there), but it also feels good to stretch out abdominals on the ball.