Suet is good. I've heard stories of people who, having run out of bird seed, decided that corn chips and cereal would be suitable. Of course, the easiest way to prevent this from happening is to stock up on high quality seed. However, should you find you are out, let the birds fend for themselves. They know how to survive these conditions, and feeding them chemical-laden products is simply an unknown but likely unwise practice.
Avian chemistry is completely different from our chemistry.
Birds need sources of water in the winter that are not frozen. If you can provide running water, so much the better. Many people do not realize that birds are quite clean, and bathe frequently, even in freezing conditions. Keep your field guide and optics handy. The fronts that generate snow can push birds out in front of them, meaning the occasional unexpected visitor to your feeder.