Lack of focus doesn't just come from external interruptions. We are also to blame—we turn on music, we have magically updating screen backgrounds, we have IRC chat rooms scrolling and instant message clients trying to catch our attention. Clutter distracts the eye, which distracts the brain. A messy desktop (both physical and on the computer) is full of distractions.
Spend a few minutes cleaning up your desk. Personally, I find it very difficult to clean my desk, so I've developed an office cleaning mantra:
When in doubt, throw it out.
I then follow this three-step plan:
File the things that can be filed.
Take the unfinished items and put them in a stack to be done soon.
Put all the remaining stuff in a large envelope marked, "If I haven't opened this three months from now, I can throw it out." Then seal the envelope.
Three months from now it will take extreme willpower to throw out the envelope without looking at the contents. The point is that I don't spend a lot of time thinking about each item and worrying that I might need it later. When deciding to throw out the envelope I repeat the following mantra:
When in doubt, throw it out. If I ever do need it, I can ask the source for a copy.
I've also found it useful to take down posters, calendars, and other things that are in my direct line of vision. I still have many posters, they just aren't in my direct view. When I'm sitting at my desk facing my computer, I want blank walls, nothing distracting.