How might one prevent testicular and prostate cancer?

Cancer Prevention. While most research of the past few decades has focused on treating cancer, there are new inroads being made in discerning how to prevent it. For the two you list, however, there are no established methods of prevention. There are correlations between high-fat, low fiber diets and risk of prostate cancer, as borne out in the increased incidence of prostate cancer in asian men who move to america.
Get checkup/selfexam. Self exam monthly and going to get you prostate checked by a professional are good preventable methods for these diseases.