I wear bifocals and for sunglasses I use these:
http://www.cocoonseyewear.com/sunwear/category.php?id=1
They look better than the clip-ons and wrap around enough to block most of the sun. They are reasonably priced and have different sizes and lenses. I think I can use them for skiing as well. This is best solution I have found so far as I don't use contacts. And progressives just don't cut it either.