Position yourself correctly.
At the beginning of each at bat, you need to position yourself accordingly -- if it's a power hitter, stay back and for a smaller guy, move in. The situation is important too. If there is nobody on, just stand where you think they'll hit it. If there are men on and you want to prevent a big play, like a bases-clearing double, move back more than you normally would.
Get a good jump on the ball.
You need to be able to see the hitter make contact, so ensure you have a good view of the plate. When someone hits a ball to the outfield, the natural reaction is to move in, but don't. You need to stay back, it's much easier to move in than run backwards.
Be aware when there is a runner on first that is a threat to steal, because covering second is the center fielder's responsibility.
If there is a throw down that eludes the second basemen or shortstop, you need to pick up the ball and hold the runner at second, or throw them out at third.
Use TWO hands to catch the ball.
This will hopefully prevent an error.