We have two closets in the master bedroom and they have wire frame shelving along one wall. I found that the typical closet shelf is about 6' off the floor leaving 2' between it and the ceiling. Divided that in half and put anther shelf above the first. Its just the right height for things like bankers boxes, ammo cans, 3 ring binders, milk crate sized file folder boxes, etc.
I found an ACE hardware that sold metal U shaped mounts for the shelving rather than the plastic expanding the drywall that most places sell. These let me mount to the studs with 3" screws making it string enough I can support my weight with them so a few heavy items isn't a problem.
For this extra layer of shelving I ran it all the way around the closet rather than just the clothing side. Then copied the same idea for the garage, ran shelving all the way around the top.