Keep the ball out of the palm of the hand. The 4-seamer is the pitch that can be thrown with the greatest velocity. The backward spin created when the pitch is released provides a true and accurate path. It is considered the easiest pitch to control. It is for this reason that the 4-seam grip is taught to every defensive player, due to its' accuracy.
Fastballs develop arm strength by building arm muscles.
You get stronger through exercise. Throwing strikes is the name of the game. Hitting is timing. If a pitcher throws the same pitch over and over again, regardless of its' speed, a good hitter will catch up to it. Using a change-up will add variety to your pitcher's arsenal and disrupt the batters timing.