This question made me laugh. Lowering sperm count is NOT an effective method of birth control. This is because it does only take 1 sperm to fertilize an egg. But if you're serious about this? Hot tubs and using a laptop as its name suggests (in your lap) (and NOT at the same time for god's sake) are pretty bad for the little wriggly guys.
you can also keep your cell phone in between your legs close to your testicles?