A recent study done on mice will hopefully show relevance in human experience, if we can get the data. Mice with high risk of breast cancer development were protected with high levels of omega 3 fatty acids.
In my mind, the best source of these fatty acids comes from eating cold water fish (herring, mackerel, cod, halibut, wild salmon), twice weekly. For supplemental omega 3's, I like Barlean's Omega 3 Swirl (tastes so good you will take it regularly) or other reputable omega 3 supplements.
An important point to remember is that your oils must be balanced so the fatty acids circulating in your body can be balanced. Eating fish and taking omega 3's won't be very helpful if you're over-eating omega 6 fatty acids (healthy version: too many nuts; unhealthy version: too many processed foods.) Your body "consumes" omega 3's to balance excessive levels of omega 6's.