If you struggle with low- or medium-trajectory pitching, this is a great drill to try. Simply take two yardsticks and place one through your belt loops and the other on the left side of your grip. Make sure the second yardstick runs up the shaft under your left arm. Take your setup and try pitching some balls with the two yardsticks in place, all the while concentrating on getting your left hip rotating open and keeping your left wrist flat through impact. The yardsticks should clearly indicate whether or not your technique is sound. Make extra-sure the yardstick in your belt loops doesn't collide with the one in your left hand.
Maintaining a flat left wrist through impact while properly rotating your hips open are keys to solid pitching.