I had a pair of prescription polarized sunglasses made this last year. They aren't quite as dark as I'd like for spending full days of full sun on snow and ice, but even so, I've spent full days on the glaciers wearing them without any problems. Polarization offers a lot of protection without as much apparent darkening or opacity. I don't really switch around much through the day: once the sunglasses are on, I'll wear them until just before dark.
It is so much easier than dealing with contacts that I don't think I'll go back. I hate trying to put in contacts at 3am when it's subfreezing out and I'm no fan of sleeping with them in (dehydrated with contacts in is a receipe for an eye infection, at about the worst possible time...) and trying to dissolve the morning eyeball superglue. A disclaimer though: I did it for over a decade without incident.
It ain't cheap, though, so see if your boss offers a 'medical savings account' - it'll cut the cost of prescripts. by about a third.
