On Lifehacker there are instructions for making snow tires for your bike. The instructions are more for road bikes but I don't see any reason why they wouldn't work (better) for mountain bike tires.
Source -> http://lifehacker.com/5719714/zip-tie-snow-tires-the-cheapest-way-to-blizzard+proof-your-bike
When I was in high school my friend Jim and I made a set of studded tires for our mountain bikes using sheet metal screws, a bald set of mountain bike tires and tuffy tire liners. The trick was to make sure the screws went through the thickest part of the tire (on a knob each time) and make sure the liner was seated over all the screw heads. You couldn't use them on anything resembling pavement, due to the screw lengths they were strictly for thick snow. See pic below for something similar to what we made.