As an outfielder, one of the hardest balls to get a beat and a good reaction on is the line drive that is hit directly at you.
This ball makes it very hard to judge the angle and the velocity of the ball, so judging distance is usually a big problem.
The best way to get used to these balls is simply practicing getting jumps in both batting practice and off of fungo.
Here are a few tips to help:
The height of the ball off of the bat is going to play a big part in whether you will open up or immediately charge in.
A ball that is head high or above, open up to the side you will need to run back on and read it, adjusting from there.
A ball that is eye level or below, you should immediately begin sprinting in to catch the ball as that ball should not carry over your head.
It is always better to open up if you are unsure and read the baseball.
Going back is much harder to do than coming in on a baseball, so if you are going to make a mistake on judgment, make it on the direction that is harder to go.
Practice, practice, practice.
Like any other part of outfield, the only way to become efficient at reading line drives is to work on them during batting practice.
The stronger the hitter, the more power he will be able to generate, making his ball carry better than a weaker hitter.
Without practicing this in bp, it will be virtually impossible to perfect for a game!