Studies have shown that testicular temperatures of 93 degrees or more can hurt the production and storage of sperm. Some people might call you crazy for icing down your testicles, but it's actually a well-recognized method of fertility-enhancement. You can buy a battery-operated machine that cools the testicles, or you can simply use frozen blue ice packs wrapped in towels. For results, you'll probably need to continue for over two months, since sperm cells need 70 to 80 days to mature.
