Here's some good articles on losing weight while breastfeeding. In general, the main thing I see is to make sure you're eating enough. Not doing so seems to be the number one reason breastfeeding moms don't lose weight.

http://kellymom.com/nutrition/mothers-diet/mom-weightloss/
http://www.lalecheleague.org/llleaderweb/lv/lvoctnov97p115.html
http://www.breastfeedingbasics.com/articles/nutrition-exercise-and-weight-loss