Taking 200 mg of Selenium has been shown to reduce cancer risks by 50%!!! You can research it for yourself if you'd like, I just can't post it on this board.
Other supplements: Turmeric and Alpha Lipoic Acid - both very potent in antioxidants that, in trial studies, have shown to reduce cancer risk and also reduce the size of active tumors. In fact, more research is being done on Tumeric to possibly market it as a an actual supplement specifically for cancer prevention.
Also, drinking 8 oz. of POM Wonderful Pomegranate juice wouldn't hurt