Throw underhand pitches to a catcher without focusing on the strike zone to start. It is easier to focus on speed first than it is to focus on both speed and accuracy at the same time. To increase velocity as you throw the ball, practice having your dominant side hip shift toward home plate as you release the ball. If you are right handed, as you step with your left foot to start the pitch, your right hip should follow through with the ball.
Pull your pitching wrist back right before you release the ball. As you let go of the ball, snap your wrist to give the ball added velocity. This technique also works in fastpitch, but it can be altered to work in a modified fastpitch as well. The faster you snap your wrist, the more velocity you will add to the pitch.
Maintain strong fluid motion as you take a strong step toward home plate, raising your gloved hand to point toward the catcher. The strong forward step gives you momentum, and as you follow through by using your gloved hand as balance, snap both your wrist and hip.