It is good to show [tag]baseball[/tag] outfielders at an early age the appropriate way to approach a fly ball – especially if there is a threatening runner on base.
What you need (set up): Set up three lines of players to indicate left, center and right fields. The coach should have a bat to hit fly balls to the players. One player may be used to snag balls for the coach. 
How this drill works: At first, you might want to start with the lines close, and just hit soft, low flies so the players can work on the footwork. The footwork for this [tag]baseball drill outfield[/tag] should be taught beforehand. It should be: back up a couple of steps behind where the ball is anticipated to drop, and keep one foot behind the other, ready to take a step forward to catch the ball with momentum.
Once they are ready to receive the ball, they should be stepping forward with their gloves prepared, and their throwing hand ready to get the ball. Just prior to catching the ball, the player should take a couple of steps into the ball to build momentum and then throw with the momentum.
It will take the players a little while to catch on, so at first you should only do the drill from about 25 feet away, and do short pop ups for them. As they gain confidence in preparing to throw right after the catch, you can move back and the fly balls can go higher.
Also, you can actually add the runner that they have to throw out at second base, or use the relay drill earlier in this section, combined with this drill to work on both areas.