As you become familiar with your own breasts, you will more easily detect any change. Examine your breasts at that same time in your cycle every month. Ask your primary care provider to teach you how to examine your breasts. Many physicians have plastic models of breasts, with lumps, that women can practice on. Finally, I should tell you that not all physicians believe that breast self-examination is a good thing.
Some physicians feel that women are too likely to be worried by general lumpiness.
Other physicians feel as I do that a women is more likely to notice a change in her own breast. Several genes have been identified that are associated with increased risk. A family history of breast and ovarian cancer, particularly at a young age, is one thing to look for. This is the first step. Next, have this confirmed, preferably at a familial cancer clinic, by a trained geneticist.