Many knee injuries happen because the person doesn't do a thorough job of warming up and letting the muscles get ready for action. Stretching should always be done before exercising or playing sports. Strong leg muscles help take the burden off the knee. When someone is active, one way the knee gets injured is through sudden change in velocity or motion. Someone who is running in one directions, stops, and then turns is putting the knee at risk. Jumping and landing hard is also very hard on the knee.
Ensuring that you wear good, supporting shoes will help your knees at well. Be sure that you are wearing the proper shoe for the activity. Finally, some people who have already injured a knee will wear a brace on their doctor's recommendation. If this has been advised, be sure to wear the brace as needed.