With all due respect, i don't want to wipe out all other suggestions, but, testicles tend to rise and move closer to the body when they are cold. The oposite happens when they are warm, or at least warmer than needed. This happens because the male genitals need lower temperature than the rest of the body in order to function propperly. If they are warm they hang low, away from any extra source of heat like the body. If they are cold, the skin that encloses them shrinks thus creating a look of smaller or tighter balls. If you sunbathe naked, the sun will warm things up and testicles will - usaually - hang lower than other times in a loose skin. Cold water on the other hand will force them (absolutely natural) to move back to the body to gain more heat. So mentioned techinques as weight lifting, stretching etc etc may harm the glands.
I myselftend to hang low and i pack quite big balls and i see no downside to having big balls.