I know it is difficult (I've given birth to 3 babies). #1 breast feed--the baby sucks all the fat away. Honestly, if you eat a healthy diet, low in simple carbs, you might just get too thin breast feeding. With 2 of my babies, I lost too much weight breast feeding and had to up my caloric intake. As for exercise, once given to OK by your doctor to start serious exercising, I recommend Zumba.
As a Zumba instructor, I can honestly report that all of those booty shakes will give you the strongest and leanest abs around--a bonus is that the moves are super for the pelvic floor muscles! Do 3 1h classes a week minimum to see results. Also, wear armor (by this I mean a super-support sports bra if you are breast feeding) :)
Accept though, that your stomach skin after 3 or more babies will always be a bit wrinkled and saggy. But I live with the stretched skin as I think plastic surgery is too big a health risk to take for a little loose skin on the belly.