Some automatic 35 mm SLR cameras have an exposure compensation control that is designed specifically for giving you correct exposure in conditions like these. For wider shots, such as landscapes, try setting the film-speed dial at half the actual film speed. This will compensate for the deceptive brightness of the snow. Remember that any time you halve the film speed, it's the same as adding an extra stop of exposure. By exposing for the background and using flash to illuminate faces or other foreground subjects, you can create a more pleasing balance between the two.
Flash fill looks most natural when the filled area is one or two stops darker than the background.
See your flash or camera instruction manual for more details on fill-in flash.