The most useful skill from the Doomsday Preppers show that I've seen (besides your typical food growing, shooting and etcetera) is food preservation.

Preserve and bottle your own food. Do it right and cooked food will last months. If you have a homemaker in the family with enough time on their hands, or if you can dedicate your weekends to it, this will also pay off with product you can sell (even without the doomsday scenario) or trade for other supplies.