Replying to Healthy Knees

So I've been venturing out of the park a bit more lately and skiing in the trees and deeper snow as we just got a huge dump in the east. One thing I've noticed is that my knees are under way more stress doing that type of skiing than when I'm in the park and they're almost always sore before, during and after skiing so i was wondering is there anything you can do without getting too serious to keep your knees in good shape? I haven't had any knee problems in the past so I'm assuming my knees are in decent shape as it is. I just want to keep them that way.