Double up the top of closets, the top shelf of a closet is about 30" below the ceiling, you can put another shelf half way in between and all the stuff you don't use very often up up higher.
Also open up an interior wall, usually side of back of closets, you then get a 3.5" deep opening which you can put in shelves and stack stuff there.
Remove the flooring of the closer, build a simple box to outline the closet floor out of 1x4 or 2x4 or 1x6 or 2x6 lumber sitting on its side, cover with plywood add a couple hinges and then recover with the old flooring so you in effect raise the floor 4-6" up with empty space under.
Though I noticed our crawlspace under the house stays pretty decent temp year round since its underground. I lined it with shelves.