From a productivity point of view, I find two things exceptionally necessary when I'm working by myself.
The first one is order. I try to keep a minimal and clean setup, because clutter distracts me, both on a conscious as well as a subconscious level. The things I keep in my desk are just the computer, paper, and a selection of pens, pencils and colors. I use paper a lot, I usually have a big sheet around the keyboard, and I write related and unrelated stuff in there. This might be of help: 3 Steps to a Permanently Clear Desk.
The second one is beauty. Beauty is a principle of design, and as silly as it sounds, I find it essential. I find beauty in symmetry, for example, so I've decorated most of my walls around the working area with my collection of "inspiring designs" (in a symmetrical pattern :D).