The first step is easy. Sit at your work area (or stand, if you work on your feet). Close your eyes. Take a deep breath. Get your head as clear as you can in the space of one breath. Breathe out. Open your eyes and let your attention fall on something within reach that looks out of place. Take a moment to deal with it, in whatever way makes sense for you in the moment. This might mean throwing something away, filing it, or jotting down a note to deal with the thing later.
Repeat this a few. Over time, increase your perimeter. Open a drawer. Look at the nearby walls, or the floor nearby.
As I do this now, I see (and move) a Land’s End catalog, some papers that need filing, an expired post-it note on my monitor, and a pair of sunglasses that don’t need to be on my desk.
The second step happens in its own time. Something you need won’t be within reach. As you get it, ask yourself if this thing needs to stay within reach. If so, take a moment and find some other thing that is within reach but doesn’t need to be, and move that other thing somewhere else. Simple rule: something comes, something goes. (Or, if you read Boing Boing, something comes to town, something leaves.)
I’m at an impasse with the second step. There are reference books I reach for every few days, but the need to have them within reach isn’t strong enough to displace anything else.
This slow, incremental decluttering works a lot better for me than big bang cleanups. The latter do nothing to avoid the clutter building up again.