Unopened balls stay in my house in a closet.
Opened but fresh balls stay in a 3-ball repressurizer (the kind that screw the top and bottom together to create pressure).
Opened but older balls get put into one of my three Cornelius keg tennis ball repressurizers and used in my tennis machine.
A Head Microgel Radical, a Neos 1k, and a Silent Partner Star. Life is good.