one thing that makes a huge difference - and it seems like no one in the medical field will ever tell anybody this - is making sure you are eating enough. the body does not know the difference btwn starvation and dieting, or just not getting enough to eat. i actually gained weight when i nursed my first, and i firmly believe that my body was in starvation mode trying to make milk and me not eating enough really even for me, much less for a nursing infant too.
try to eat a well-balanced diet with water, vegetable, grain, protein and fruit at every meal, and snack every couple of hours on vegetables in between meals... don't go more than two hours without eating at least a serving of veggies, and be sure to drink plenty of water, too.
there's a great book called Lean and Free 2000 Plus
by Dana Thornock - you can get it on amazon.com marketplace for around $7 incl shipping... she's a nutritionist and has been able to stay lean and trim and very healthy eating not less than 2000 calories per day, and she also nursed her babies while eating on her plan. it's VERY easy and very healthful, and so cheap it's easy to afford to take a look at it.
if you're making sure your body is getting enough nourishment now, really getting the calories and nutrients it needs, and you keep that up after baby is born (so hard because we're so busy and sleep deprived!) it really ought to help.