One of my friends was over weight, and she had a C...
She worked out... nearly daily, with a personal trainer... who pushed and pushed... She changed her diet and basicly was well on the path to becoming a female body builder... she did not take steroids...
After about 7 or 8 months of the intense workouts she was flat... and still is flat... an AA <shes jelous because I have a better bust LoL>
Anyways, with the all the working out, a combonation of cardio, weight lifting, low fat, low carb diet she got to the point where she was looking very manish... and by very, I was more passable than she was <and I was still in the early stages>
this may not work for everyone, but its worth a shot... This may not work all that well for those who are overly skinny though because there breast may be more breast tissue and less fat to begin with.