Body contact is an important part of hockey that can also be dangerous. Learning the proper way to deliver a check can save both you and your opponent from possible injury. Using your stick to knock the puck away is also an option that can avoid body checking altogether.
Balance
It is important to be balanced on your skates when you deliver a check. Try to distribute your weight evenly on both feet and make contact with a strong foundation. Working on your skating skills will help you stay on your feet when you check opponents.
Speed and Angles
To reduce the likelihood of injury to you and your opponent, try to gauge your speed so you are skating about as fast as the other player when you deliver the hit. Avoid hitting him head-on. Instead, use angles to direct him into the corner or the boards. This will help prevent injury and give him less ice to work with.
Technique
When you check another player, drive through the check with your legs, keeping your knees bent as you make contact. Keep your hands at chest level and your elbow down, leaning into him with your shoulder.
Stick Checks
If the main purpose of checking is to separate an opposing player from the puck, you will find many instances where you don't need body contact at all. Using your stick to poke the puck away is perfectly acceptable and will conserve energy. Examples are a poke check, where you hold the end of your stick in one hand and poke the puck away, and a sweep check, where you use a sweeping motion with the stick.
