Screw a sturdy curtain rod into a sloped ceiling, running the curtain rod parallel to the floor. Test positions for the rod, and ensure that you can hang clothes on the rod without the clothes hitting the floor or the down-slope of the ceiling. Adding a curtain rod allows you to hang your clothing instead of storing them in boxes.
Place a tall credenza on the wall where the sloped ceiling ends. A tall credenza has shelves and drawers that you can use to maximize your storage space while maintaining some simple accessibility to the items stored. The credenza also helps to pull the visual space away from the corner where the ceiling actually meets the wall, allowing the room to look taller.
Line the wall below a sloped ceiling with long shelves. The shelves allow you to display the items your storing and keep them easily accessible. Long shelves also can hold small boxes neatly and without a sense of visual clutter.