The mechanism to reduce body fat using the body is described by physiological research for quite some time. So lose weight without losing breasts is unlikely, since it consists almost entirely of fat.
The breast is made up of ducts, mammary gland and fat. For that reason, lose weight without falling breasts is quite difficult, precisely because this adipose tissue within the breast volume granted; diet and exercise do decrease the fat of the breast , and these are reduced.
In other words, if you decide to lose weight by exercising, the route will reduce visceral fat is first, followed by chest, arms, back and abdomen. Breasts are part of the chest, so the fat present inside of them will decrease and the bust will reduce its size.
The alternative is a medical treatment; the more effective is liposuction, which models any part of your body as you hope, as it eliminates honestly, during a mechanical action, adipose tissue. Anyway, it is a very complex decision, since it is a surgical procedure that involves risks.
In short, the area treated with liposuction, generally resistant to the reducing action of diet and exercise, the amount of fat tissue. However, after surgery you have to keep taking care of yourself, because if you gain weight, fat is distributed in other areas not operated, they can gain more volume than before.