For non pros, one thing that may be of a concern is 'who pays for the balls' after practicing with your playing partner. If this question is not addressed front on than we may see old balls being used every time which is not very pleasant. I think that the best way to deal with this problem is to play a match (one game, a few games, a set etc.) near end of the session and the loser pays. Not only does this solve the ball problem and being able to use new balls each time but also the better player on the day can be rewarded. Moreover, this will also inspire better tennis from both players. What do you think and how do you deal with this problem?