There are some great pieces of advice here, no doubt from years of trial and error. The orginal question is great and in fact I've been reading a great book on the matter lately. I'll post the link below. (It's related to tennis mental game, but same principles apply to baseball.)
Let me offer a couple things I work with my players on:
Picture yourself standing on the tee box at your favorite golf course, or at the free throw line in a basketball game. Most of us have experienced one of those scenarios before. What happens when you begin to think about the mechanics of the next step involved in your action? Your inner voice may sound something like, "Ok, this time you just need to work on bringing the club head through the ball on a straight plane. Oh, and dont forget to follow through after contact!". At the freethrow line you may hear, "Remember, this is a big shot. You're down by one and you need to sink this shot to give your team a chance. If you miss this shot, coach will certainly be furious and you'll probably run more at practice Tuesday." Most of us have experienced something similar in these situations. Usually when this type of inner voice clutter gets too loud or too frequent we struggle with our performance. We shank the ball or miss the free throw.
It's interesting to think about the conversations that take place in our minds on a daily basis. In any conversation there is one party doing the talking and one party doing the listening. This implies that you have two selves conversing ideas and thoughts in your mind while you are trying to perform a task. The author below calls these Self 1 and Self 2. Self 1 in is charge of telling (mechanics, directions, commands) and Self 2 is in charge of doing (muscle memory). Great athletes are those who can silence Self 1 during performances and rely on Self 2 (muscle memory) to take over.
Here's how it works. Imagine that Self 1 and Self 2 are actual people for a moment. In baseball, Self 1 would be talking to Self 2 saying, "Remember, you need to get a proper weight shift to create power on this next pitch! And you do know last time up to bat this guy blew the ball right past you because you weren't ready! But, you can't forget about his curveball, it's nasty and you probably can't hit it!" Self 2 all the while is trying to focus on the job of hitting the baseball with Self 1 screaming commands in his ear. It would be tough to concentrate and over time, the confidence of Self 2 would only diminish if he could not perform under the pressure Self 1 was creating. Of course, this would only increase the amount of commands and directives Self 1 would issue to Self 2 in the attempt to get him to fix things.
I think you get the point. So now the question is, how do you shut up Self 1 when you are trying to focus on performing a highly precise task of hitting a baseball consistently?
In order to keep Self 1 quiet you need to learn how to keep him occupied. Below I provide one good way to do this. But, just like working on your hitting mechanics, developing the mental game of baseball takes repetition and practice.
Breath. If you can focus on listening to your breathing you will keep Self 1 quiet. This will simply allow Self 2 to do his job without interference because Self 1 will be occupied with demanding focus on listening to your breathing. When your body and muscles are relaxed, your breath is slow and rhythmic. When you are tense or distracted you tend to have more of a quick breathing pattern. By listening to your breathing you will also allow your body to relax and become in synchronization with your breathing. Controlled breathing is a great way to allow Self 2 to perform to the best of his ability.
Have a read, I quite enjoyed this book - Timothy Gallwey - The Inner Game of Tennis - http://www.amazon.com/Inner-Ga...rmance/dp/0679778314