If you find out, you could make a fortune. Much of the breast tissue is made of fat, and it's nature's cruel joke that we should work out to improve our bodies and this is the FIRST place we lose the fat.
I've recently seen an ad in the various muscle mags for some special cream that supposedly prevents fat loss from breasts, but it's setting off my "snake oil detector" and I'm holding onto my wallet. I haven't heard of anyone who tried it (and wanted to admit it).