Stepping out generally begins because of a fear of the ball and may continue later simply because of habit. To help correct this habit, place a ball glove or something flat to the left of the hitter’s stride area (for right-handers), so they know when they are stepping out.
If the hitter realises they are stepping out and continues doing it, they may have a balance problem. Have them lift their front heel of the ground during their stance and stride. This will help them keep their weight forward toward the plate.
Stepping out may also be due to the desire to pull the ball. Discourage strict pull-hitting. The hitter should develop the mental image of hitting the ball straight back at the pitcher and hitting to all fields.
If fear is a factor, it is important to convince the player that he is safer when striding straight at the pitcher than when bailing out.
The proper movement of the batter when a ball is thrown at him is to turn inward toward the plate and then toward the catcher (while dropping his head if the pitch is high). This protects his head and chest (the two dangerous places to get hit). Bailing out opens the hitter up and usually results in exposing the chest and head to the ball.
Also, I know several young players who have overcome their fear through on-deck prayer.