Amazingly, slow pitching works well against big hitters. Slow pitching worked great for Jamie Moyer in the big leagues, so it can also work well in youth leagues.
At a recent 13U tournament, the small guys locating slow pitches in the zone kept big hitters off balance and swinging early.
The big power pitchers over threw, hitting the backstop or bouncing the ball in front of the plate. No pitchers dominated the game, but the slow pitchers forced pop ups and easy ground balls to get hitters out without much effort.
Most hitters spend a lot of time practicing to hit the fast ball and don't spend much time working on hitting slower pitches. In game situations, they slow down their swing trying to make contact with the slower pitch. Swinging a bat slowly will result in a weakly hit ground ball or popup for an easy out.
Remember to locate pitches in the strike zone. Work in some off speed pitches including the change-up. Rely on your fielders to make the outs. Keep moving the ball around the corners of the strike zone.
Throw low in the strikezone to induce grounders. Keep the hitter guessing by changing speed and location.