Outfield

Similar size, seams,
throwing motion.
Coincidence? I think not.

Here's a serve warm-up activity flexible enough for any number of players.

• Divide players into two more or less even teams. 
• Have one player begin by throwing a ball overhand across the net. 
• Ball must land in singles court. Ball must be caught in air or on one bounce. Player catching ball is next to throw. 

Points are awarded based on errors, just like in the real game. For example if the ball lands outside the singles court, or is not caught after one bounce, point goes to team not making the error. You can work to a specific point total or for a set time period. This game helps develop court sense - where should I throw this ball?? - and warms up the shoulder for serving.