I bought some used file cabinets and file by category, such as blouses, coats, men's, etc. One gallon or two gallon bags for those that tend to expand after you remove them from the original folds. Sometimes I put them in a plastic sleeve or page protector, then put a pin in the open end to keep them from sliding out when being handled. Most of my patterns are traced from a master one, so that makes for more to store.
Craft and quilt patterns are another story, everywhere! They started out in labeled magazine files, plastic and cardboard. Sometimes I can find one and sometimes just give up until it finds me. I've put a lot of these in notebooks from Costco or thrift stores.