I don't think about baseball at all. The whole day at school, the car ride to the game, I never think about it. (Except for the amazing early dismissal time we all snuck onto our early dismissal slips =P)
But otherwise, it's nothing. During the car ride there I listen to From Autumn to Ashes. Yeah it's screamo and really intense, but it helps ease my mind.
I always smack my spikes against each other before I put them on, regardless of dirt.
I joke around with one of the other juniors on my team, he's really good at helping relax the head because he tries so hard to be serious but it doesn't work.
Uh, before an at-bat I never use a donut, I always use three bats of equal size. At the plate I step in right foot, dig in and lay the bat across the plate with a thump to make sure I'm in the right spot.
And before I pitch if I throw one in the dirt to my coaches dismay, I always yell "Chill, it's a warm up." I think that helps me stay relaxed.