This is a question I've been asking myself over and over since I first heard the diagnoses. The Mayo Clinic does list things you can do to limit the risk.
Limit alcohol - I'm not a heavy drinker, though I suppose completely abstaining from alcohol could have reduced my risk.
Control your weight - Controlling my weight has always been a challenge. I'm a stress eater (though in the last year and a half, I've found other ways of dealing with stress. I'm convinced that having lost the almost 80 pounds made it easier for the cancer to be detected early. I also believe it made the recovery from the surgeries much easier.
Get plenty of physical activity - I'm fairly active now. But prior to June of 2010, I wasn't. Too busy sticking my nose in a book to get much exercise.
Breast-feed - nope, didn't do that either. Over the years, I've often wondered if I'd done my boys a disservice because I didn't breast feed. It never occurred to me that breast feeding might have been to my benefit. I used to joke that my chest was so large I was afraid I'd smother the boys. I used to want a "boob job" - but not what you think - I wanted a reduction. Considering that if the cancer had not been detected early I would have had a drastic reduction, I've decided to be happy with what God gave me.
Discontinue hormone therapy - Never had hormone therapy until after the cancer. Tamoxifen is considered hormone therapy.
Avoid exposure to environmental pollution - The Mayo Clinic states "While further studies are needed, some research suggests a link between breast cancer and exposure to the polycyclic aromatic hydrocarbons found in vehicle exhaust and air pollution." (http://www.mayoclinic.com/health/breast-cancer-prevention/WO00091). Since I've never lived in highly polluted area, I guess this, avoiding hormone thereapy and the limited alcohol are the only preventatives that I actually managed to follow.
Eat a healthy diet - A diet rich in fruits and vegetables doesn't offer direct protection from breast cancer, but it can help you maintain a healthy weight, which is a key factor in breast cancer prevention. But again, though I consider my current diet to be healthy, it's only recently become healthy. Eating healthy is not easy nor cheap. Why is it that food that is good for you is so much more expensive?
If I had followed all these precautions would I have gotten breast cancer? Since I don't have a crystal ball to tell me what God's plan was, I can't answer that.
Some of these things I failed to do because I was young and stupid. If I could go back in time retaining the knowledge I have now, I would probably follow these precautions, but hindsight is always perfect, isn't it?