First, be aware of your feet. If your feet are inverted you'll end up missing pitches on the outside corner. Most umpires who struggle with left handed batters have their feet inverted.
Second, make sure you are tracking the ball and don't have tunnel vision. Lie down on your bed and throw a rolled up pair of socks in the air and track it with your eyes.
Third, check your timing. Make sure you let everything that can happen, happen before you decide if the pitch is a ball or a strike. See the ball all the way to the plate, read how the catcher catches it, and then make your decision.
Hope this helps you out.
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