Where does your desk face?
If you work with other people, unless you're a receptionist don't position your desk facing the flow of traffic. People walking past will catch your eye. It is a natural reaction to glance up as footsteps approach and once eye contact has been made your chain of thought has been broken - even if you don't speak.
An Information Technology manager for a very large international bookstore franchise decided to do something about his endless interruptions. Problem was, his desk faced swinging doors through which people walked many times a day. Even though he was a quiet man who preferred to concentrate on his complex work, he was just the same as everyone else - wired to look up when someone came toward him.
He turned his desk 90 degrees away from the door and positioned some bookshelves to block the view. What he now saw were the backs of people as they walked away from the entrance. And when they exited they weren't looking directly at him. Outcome? With excitement he informed me that he'd dramatically reduced his interruptions; this equalled about an hour a day of extra productivity.
If you've got fixed furniture that can't be moved, these ideas might help: