There are few things in baseball pitching that don't directly correlate to the quality of a pitcher's mechanics. Want to throw harder? Improve your mechanics. Want to throw more strikes? Improve your mechanics.
Pitching command is the same. The command you have of your pitches and the command you have of the strike zone work in concert with the quality of your motion. The better you are able to consistently repeat your pitching delivery, the better you'll be able to command your pitches.
You need to try and learn to hit the plate, consistancy and command is the key. When you learn to keep it in the zone then you can learn to increase your velocity as you gain more and more command.
Visual cues to improve command
The biggest help for better pitching command in my book is "Visualize it, verbalize it."
Here's how it works:
You're a righty and you need to paint a fastball on the outside part of the plate. Visualize it. See yourself making that pitch. Go ahead: really see yourself making that pitcho to that location. Then, verbalize it. Say out loud: "Nice and easy, fastball outside." Repeat it throughout your delivery, too. Say: "Nice and easy, fastball outside."
It works for all pitches in all locations, and it's great because it really gets you thinking about your pitch and location. That way, every pitch you throw is with conviction, to a location and with a purpose.
(In games, you'd "verbilize" your pitch and location under your breath so as not to tip off your pitch, of course.)
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