Most of my family's bikes live in the garage, 7 in a rack I made from a crib, and two hung from the ceiling. But that leaves 4 bikes that are rarely used that we keep out back. To protect them from sun/rain damage, we cover them with a regular plastic tarp.

Originally I bought some cheap (few dollar) plastic rain covers off eBay, but they were so thin the handlebars tore through them first time we tried to cover the bikes up. So make sure if you are buying covers, they are good quality. User reviews should be helpful.