Tim:
If you have snow, that is the way to go. There is no sense in attempting to keep dirt sets going and the have them fail anyway. Wrap your traps loosely in crushed waxed paper and cover them with snow. Sift the snow over the set from up high in the air so the snow can collect air on its way down. This keeps the snow from freezing hard over the trap. If you use a zip-loc bag, try it out first as some may not break open easily. When this happens one may find a snapped trap with a ball of snow between the jaws or the trap speed was hampered and the jaws closed too slowly, causing a complete miss. When using bags it is best to take a knife and perforate the bag along the open jaw line. Once you have made the set, back away carefully brushing out tracks back a reasonable distance. This is more important with canines than with cats but one should pay attention to detail regardless of the animal persued. It is always the best policy. Good luck! Ace