just throwing this out there since I personally deal with this and I work for a PT now. .. Make sure your muscles are firing correctly before you ski. You would be quite surprised by the number of people who's hamstrings and glutes dont fire correctly putting a ton of stress on knees and quads. all the lessons in the world won't fix this. its the same for cycling too.
a quick and overly simplified test is to do a one-legged squat, you only need to go down to 30-45*. see if you can feel your muscles firing throughout the entire eccentric and concentric movement. go slow. Also, do some forward lunges on each leg, make sure you can feel the glutes and hamstrings firing correctly through th entire movement. 3rd, on your back, do 10 or so bridges, first with 2 legs, then try some single leg. I do sets of these almost daily and will do a few of each before skiing and cycling.
do not attempt if you have any back problems.