The purpose of cleaning your office is to help you get organized. The best place to start is at the part of your office where you spend the most time -- your desk.
Is your desk buried under stacks of papers, rainbow-colored mountains of Post-Its and piles of office supplies? Start with a clean slate. Take everything off your desk except for your computer, printer and phone.
This might be the first time you've actually looked at your desk in a long time. If what you're looking at is several years' worth of accumulated dust, you can use this opportunity to clean your desk and vacuum under it.
Then, go through every item that was on your desk. Consider how often you use it. Once a day? Once a week? Once a month? Never? If you don't use the item at least once a week, put it aside. Once you've organized the rest of your office (read further down this list), you can find new homes for all of the supplies and other items you've relocated.