Came across a good online guide about stretching by a Canadian physical therapist and acupuncturist. What's unusual is that she includes references to clinical studies along with easy to understand explanations about a variety of stretches. Unfortunately there isn't a section specific to skiing even though she skis and snowboards.
I used to be most interested in hamstrings for good knee support, and calf muscles. The next area I plan to work on are hips. As my ski technique has improved, the importance of ankle and hip flexibility has become more understandable.
A great reference that I found a few years ago is The Anatomy of Stretching, 2nd edition, 2011, by Brad Walker. The illustrations show specific muscles in the context of the rest of the body. Each stretch is explained clearly and simply, and there is a list of sports and injuries for which the stretch is useful.