Studies (here's one) have shown that women tend to overestimate the risk of breast cancer and underestimate the risk of other things that are more likely to kill them--such as heart disease.
But some women really are at substantially increased risk of breast cancer. Specifically, women who have certain identified mutations in the BRCA1 or BRCA2 genes are much more likely to develop breast and/or ovarian cancer. These mutations are rare in the general population, but one expert group recommends genetic testing for women with 2 0r more relatives with breast cancer before age 50, or 3 or more relatives with breast cancer at any age. (deBock et al, 1999).
If a woman has one of the mutations, she has a few options. One is to have more frequent, and more intensive, screening. Another is to have prophylactic surgery: removal of the breast tissue (mastectomy) and/or oophorectomy (removal of the ovaries). In a new book, Pretty Is What Changes, author Jessica Queller tells her story, which began with her mother's death from ovarian cancer and her own decision to have a radical mastectomy in her mid-30s. You can listen to an NPR interview with the author, and read an excerpt from the book, here.