You see it in every single game - the pitcher delivers, the batter gets a little under the pitch, and look out! The hitter has popped it almost straight up. If the catcher makes the play, the out is nothing short of huge... but many times, especially in the younger levels of youth baseball, the ball will fall to the ground within easy distance of the catcher, and the batter suffers nothing more than a strike.
Take five or ten minutes per practice early in the season, and your team can easily realize a few exra outs over the course of the year.
The drill itself is simple. Get your catcher in full gear, and have him squat in his stance. Have a coach stand a couple of feet directly in front of the player with his back to the catcher. Give a signal (I like to have the coach yell "ping", like the ball coming off of the bat) and toss the ball directly up into the air. Obviously, the object is for the catcher to keep the pop up from hitting the dirt.
Make sure to emphasize the importance of ripping off the mask in the same motion as the catcher comes to his feet - if the mask stays on, the chance of catching a pop are drastically reduced. He should be looking up, and balanced, in order to move to the ball. Have the coach vary the location and height of his tosses - a catcher that can effectively cover not only the area between himself and the pitcher, but can also patrol the foul areas well, is a huge defensive boost to your team.
Don't forget that, if the catcher makes the catch (even in foul ground), runners can advance at their own risk, so don't let your catcher mentally "check out" just because he makes the play!