I took Tamoxifen. Taking 5 years of tamoxifen lowers your chance of recurrence for at least 15 years (as I'm sure you know, mastectomy cannot remove 100% of breast tissue). The side effects aren't fun, but they are manageable.
My oncologist also recommended eating organic as much as possible. Also, eat something black every day, eat berries every day, have a cruciferous vegetable every day, and have at least one cup of green tea every day.