Top pitchers are able to tunnel their pitches for the first 20 feet after release, thus making it very difficult for hitters to guess the final location of the pitch. Watch the videos by Perry Husband.
Tracking a pitch - Hitters determine the direction of the pitch in the first 20 Feet of Flight
Matt Harvey 20 Ft Pen
Try this strategy: Place a target on a net and throw all your pitches to that location from 20 ft away. This will help with learning how to tunnel the pitches.