Practice practice practice!

balance is the most important thing and comfortabilty is key relax and dont tense up

Make sure your mechanics are sound. Practice often, and strengthen yourself physically. Mental practice is essential. Mental toughness will get you through a tough inning or will help you maintain your focus all throughout the game.

check this out:
http://www.softballperformance.com/blog/6-mental-training-tips-how-to-improve-your-mental-toughness/