Human Kinetics at www.humankinetics.com
has a lot of good educational material. For skiers a very good tape is "Stability Ball Training vol. II" (Legs, Hips, Core). The tape has scores of exercises. Just remember, anything you can do with two legs with a physio ball, you can do with one leg. My program has 20 physio balls that are used regulary in conditioning. One especially good exercise for skiers that I have never seen demonstrated elsewhere is as follows: Stand beside the ball with one foot on the floor and the other on the top of the ball. Time yourself for 60 seconds as you push your foot into the ball as hard as you can. See if you can apply more force into the ball, than is into the floor with your foot that's on the floor. The harder you press into the ball, the more your leg will oscillate, as the knee everts and inverts very rapidly. This isometric exercise is done to help train small correcting muscular contractions to help stabilize the knee. Done frequently, you will definitely help improve finer muscle movement patterns as it applies to control of the knee and leg. Keep your upper body tall, not bent over while doing this exercise.