Although you can do a lot to firm up the underlying muscles, which will alter the appearance of your breasts, to specifically lose weight in one area of your body is nigh-on impossible. Yes I know there are all those "thigh diets" out there but they make you lose weight in general. Simple answer: use more calories than you eat. Cut down food, stoke up exercise and the weight will come off, but all over. Some people lose from different areas at different rates, but there is no predicting this.
Liposuction or breast restructuring is about the only sure fire zoned reduction, but this is a bit drastic and, frankly, rather you than me.