Guest bedroom closet - that which can be hung on hangers is. Each costume gets it own plastic tub for the parts that can't be hung on a hanger, accessories, shoes, etc. The tub is labeled with the name of the costume. The tubs are stored in the same closet.
On the inside lid of each tub is a list of all the parts of the costume needed to wear the costume, including parts on hangers, props, makeup, undergarments, etc. Makes it very easy to pack for cons.
The only thing I haven't figured out how to store is the Wicked Witch of the West's hat. It doesn't fit in the closet and it is way too wide to put on a shelf.