An artoicle in the London Daily Mail tells us that a good diet and exercise help to cut breast cancer risks:

.......walking briskly for half an hour and eating five portions of fruit and vegetables a day halved the chances of dying from the disease, even if a woman was obese.

The Health Talk Breast Cancer Blog says of the diet:

Things like a diet low in fat, low in red meat, but concentrated on more vegetables and high fiber foods are highly recommended.

As for exercise, the Science Blog says:

Three hours of moderate exercise per week significantly reduced circulating estrogens in postmenopausal women, according to a new study published in the current issue of Cancer Research. The finding may explain why women who exercise regularly lower their risk for breast cancer.