Standard practice is to Keep them in a cool and dry place, away from direct sunlight. You can use airtight containers to store tennis balls. These containers help maintain internal pressure and keep them from becoming flat or losing their bounce. Also, keep your tennis balls neat and clean as dirt can also affect their performance in the long-run. Though it's not so noticeable, care is better than treatment. You can simply wipe the balls with a cloth before storing them.