I have a wide, narrow closet where the sides extend beyond the doors. I put 3/4" molding on the three sides of the walls and use Masonite for the shelves. I can store watercolor paper and foam-board flat.

I also make large cardboard portfolios to store flat work upright behind dressers and under beds. (Make sure the work is dry!) Duct tape and cardboard are great materials for large, flat portfolios. Many appliance and furniture stores will be glad to help you out with discarded boxes.

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Thanks, Joe. Those are both good ideas. I advise protecting the work in the cardboard portfolios with acid-free barrier paper of some kind so the acid from the cardboard won't migrate into the artwork.
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Here's an idea that doesn't require hardware. This six-shelf setup is very stable, but you may want to secure it with hardware if you have small children around. If you like to play with building blocks, you'll love this one. It can be built in a closet (refigure the measurements to fit your closet) or stacked on top of a table or two 2-drawer filing cabinets. The directions are for Imperial size 22"x30" papers and boards. Adjust measurements for larger papers.

For the filing cabinet installation you need:
1 sheet particle board 4'x8'x3/4", cut into four 3'x2' panels and one 2'x4' panel (This is the base over the filing cabinets.)
1 sheet particle board 4'x4'x3/4", cut into two 3'x2' panels and one 1'x4' shelf (optional)
3 2"x4"x8' studs, cut into 2-foot lengths

Place the two filing cabinets so their outside edges are 4 feet apart. Lay the 2'x4' panel lengthwise across the cabinets. Place a 2' stud at one end and 1' from the other end. Stack a 2'x3' shelf on the two studs. Continue stacking studs and panels, ending with a panel.

There will be an area at one end for books or other items and additional storage space between the cabinets underneath the shelves. Use the 1'x4' panel as a wall shelf if you have space for it. You can buy brackets for the shelf at any home-supply store.

You're welcome to visit My Studio.