You can never have enough shelving in a studio. Install salvaged cabinets and shelving. The shelving should house art books, crates of supplies, and printer paper.
Store works of art by framing them in archival-quality mats and frames, and hanging them on the wall. The artwork will be free from bangs and nicks and can act as a constant source of pride and inspiration. They are also ready to sell at a moment's notice. You can drape a black cloth over the work to protect it from UV damage.
Hang a large bulletin board on a wall to keep track of notes, pictures and sketches.
Use plastic crates to organize bottles of paint, framing tools, and other supplies.
You can hang wet watercolors from a closet rod with hangers that have clips on them.