A question for all you skinny Mums with your washboard abs?

How did you get your stomach so flat after giving birth? and how long did it take? My second daughter is 3.5 months old, I have lost all the baby weight but still have a spongy tummy. Im sick of looking 4 months pregnant. I exercise, feed and eat pretty good (ok ok I admit I do like my choclate fixes). I need some tips on shreinking this belly in time for summer. Thanks