I always carry the palm of my left hand with me. I know from comparing to a gray card that it's 1 stop brighter than medium gray. So in difficult lighting, I put the palm of my hand in the same light as the subject and then open up 1 stop from the recommended exposure. I'd bracket in half-stops over what seemed an appropriate range if the lighting were very contrasty.

Here's the Ansel Adams rundown on snow:

Shadows on snow in sunlit landscapes Zone VI - open up 1 stop
Average snow with acute side lighting Zone VII - open up 2 stops
Textured snow Zone VIII - open up 3 stops
Snow in flat sunlight Zone IX - open up 4 stops

These all assume that you're spot metering off the snow itself under the given conditions.

See my gallery for a woodpile in the snow. I metered with the camera's spot meter and opened up 2 1/2 stops from the sunlit snow reading. Reflected light from the snow was filling in the shadows, so the contrast range wasn't that excessive. Donald's advice is good for flatter lighting than I had.

Hope this helps.