I had the same problem, although I am not as well-endowed. I bought a really heavy-duty sport's bra, one which had very little stretch to it and wore it for archery. It took care of my problem. I also found that I had the problem more when I shot a longbow, rather than a recurve. I'm not sure why, but it sure seemed that way. If you can't find the kind of brassiere that I mentioned, you might try binding your breasts.
Hope this helps,
Lyneya de Grey
Well-behaved women Seldom make history - Laurel Thatcher Ulrich