Study after study has found that women over 40 who weigh close to the same amount that they did in their 20s are less likely to get this disease. Ideally, you should gain no more than 10 percent of your body weight (so if you weighed 120 in college, you shouldn't gain more than 12 pounds over the subsequent decades).

Several studies have looked at whether fruits and vegetables are protective. According to Rock, it's vegetables, not fruit, that seem to have greater benefit. "One pooled study, which was data from several countries, showed that eating a lot of vegetables seemed to lower breast-cancer risk in all women -- and young women in particular," she says. Why is produce so beneficial? Vegetables are a very good source of fiber, which in animal studies has been shown to lower levels of estrogen circulating in the blood. Also, many veggies contain cancer-fighting phytochemicals. "The more you eat, the better," Rock says. To reap the breast benefit, get a minimum of five servings a day.

"The more exercise is studied, the clearer it becomes that physical activity protects women," Rock says. The only thing that isn't clear is how active you have to be. While studies suggest you'll get the most benefit if you get vigorous exercise at least three times a week, more-moderate amounts still seem to be helpful. "There's a good hypothesis on why it helps," Rock explains. "Women who exercise on a regular basis have lower levels of insulin and insulinlike growth factor. These anabolic hormones promote cell division; when cells are constantly dividing and growing, there's a danger something will get pushed down the road to becoming cancer." High levels of insulin and insulinlike growth factor seem to act as fuel, possibly helping cancer take off. Exercise also helps by lowering the circulating level of estrogens, Rock adds.

"Many, many studies have found a link between alcohol and breast cancer," Rock says. "But the risk does not get significant until about two drinks a day. You can still drink -- just don't overdo it." One interesting caveat: Studies in the United States and Australia have found that women who drink but also get adequate amounts of folate don't have a higher risk of breast cancer. So if you tend to enjoy a glass or two of wine with your dinner on a regular basis, taking a multivitamin every day may be a wise idea. Even better, chow down on good sources of folate: spinach, romaine lettuce, broccoli, orange juice and green peas.