There are a few theories. One is that estrogen-fed breast cancer, which is quite common, is caused in part by too much estrogen exposure in the body. This could be due to: early menses, late childbearing, lack of breastfeeding, late menopause, lack of exercise, drinking alcohol, hormones in the food supply, and a host of other things that could cause a woman (or girl) to be exposed to estrogen. So one theory of prevention is if women (and girls) reduce their exposure to these estrogen-causing things listed above (some of which we cannot control, you will notice), that it might help in reducing their chances of getting breast cancer.
Another theory that is espoused by Dr. Susan Love (one of my heroes) is that with cancer, the "neighborhood matters." Dr. Love feels that cells need to be in a certain "neighborhood" in order to overgrow.
My usual metaphor for this is thinking of a neighborhood where the individuals in the neighborhood with their own talents, neurosis, and problems (the actual cancer cells) interact with each other and also relate to the conditions and other people in the neighborhood i.e. graffiti, garbage on the street, drug pushers, crime, gangs etc. All the factors combine together to lead to bad things happening.You can read more about her theory here. This theory isn't really helpful right now, but may be helpful in the future in trying to figure out how to prevent breast cancer or how to prevent cancers from metastasizing.
Another theory, and one that is gaining traction in the scientific community, is that there are chemicals in our environment -- in the food we eat, in our homes, in our cleaning and personal care products, everywhere -- that haven't been properly tested for their impact on humans, and which are causing breast cancer as well as other cancers and other health issues as well. It's like we are living in a giant experiment. I truly feel that some day, say 50 years from now, people will look back and say: how stupid they all were! Why did they think it was okay to use pesticides on crops and give hormones to animals and then ingest that food? Why did they think it was okay to pump chemicals into waterways and into the air, so that animals and humans breathe and drink the contaminated water and air? What were they thinking???
People always want to know what they can do to prevent breast cancer, and some people take the personal approach. They choose to eat organic food, to buy organic personal care products, to avoid using pesticides in their yard, etc. This is a fine approach, but I feel that it's just not enough. We are exposed to too many chemicals just from living in our environment. Unless we live in a bubble, we can't control everything. You can control what you do, but what if your neighbor uses pesticides on her lawn? What needs to change is the policies that allow all these chemicals out into the world.
This may not be a popular position, but I'm not thrilled with websites that tell you personally how to reduce your risk. For example, the Breast Cancer Fund's website has a section called Reduce Your Risk in which it lists all kinds "tips on how you can make simple changes to reduce your risk." The Silent Spring Institute has a new website called Too Close to Home that similarly talks about how individuals can reduce their risk through avoiding chemicals and other exposures in their homes. I just don't feel that this is enough; I also feel like it's giving people a false sense of control. It's much bigger than what each of us does personally. It's much, much bigger than our personal decisions and behaviors.