Strong thigh muscles will help keep the knee in position. Doing half squats or using a leg machine will work the thigh muscles. Running hills and stairs will strengthen both quadriceps and hamstrings. Riding a bicycle three times a week either indoors or outdoors will help. Make sure that the seat is high enough to avoid excessive knee bending.
Water aerobics is also a great way to strengthen joints without a lot of stress.
The anterior cruciate ligament is the main guide to knee stabilization. Fortunately injuries to the ACL are now much more treatable and athletes are returning to performance at a greater rate. All athletes need to be aware of the risk of ACL injuries but they also need to know if it does happen, it's not the end of their athletic career.