Start training to be a switch hitter when you are young. The longer you wait to become a switch hitter the more the habits of the swing are ingrained. When you first start playing the game is the best time to learn to hit from both sides of the plate.
Practice switch hitting as often as possible. The only way to become a good switch hitter is to see and swing at thousands of pitches from both sides of the plate.
Bat right-handed against a left-handed pitcher and vice versa. The reason you do this is that it is easier for you to see the pitch coming towards you if the pitcher is throwing from the other side of the plate as you. This increases the odds of success and gets you more comfortable with being a switch hitter.
Concentrate on hitting specific pitches from each side of the plate. Depending on skill level, some players may have trouble hitting a pitch, such as a tailing fastball, from the left side of the plate. Therefore, during batting practice have the pitcher throw that pitch to you but from the right side of the plate. Pitchers usually have certain pitches they like to throw. Then when you face them, you can bat from the side of the plate where you hit his or her primary pitch the best.
Use the same hitting stance when hitting from either side of the plate. This makes it easier for you to make the transition to switch-hitting. Now later on you may need to adjust the stance once you become comfortable but try to keep the stance the same when you are first training to be a switch hitter.