Cuadricep excercising in every way (Strenght, Reaction, stretching, etc)Knee and leg polyometrics and awerness (equilibrium with balls and propioceptive excercises in general)
Warming up of knees before every session (doing circles both ways keeping knees together, bike)
If necesary just wear knee braces.
While skiing avoid bump and jump landing with straight knees, if you can...
hope it helps