Don't put your desk directly in line with a door or under a beam. This will put you in the line of Sha, or bad Qi (bad energy) so reposition your desk! If this is not a feasible option for you place a potted plant in front of your table to soften the blow of the Sha.

Don't keep your files near the bathroom or on a wall that is shared with a bathroom!
Don't put mirrors in your office, as they can reflect Sha (or Qui, bad energy) from clients or other people in the room. Also, mirrors inappropriately positioned can be very bad...they encourage you to look back on the journey of life when you should really be looking forward.
Don't forget: Your office floor is someone else's ceiling...but your ceiling is someone else's floor...and on shelves, position your books with the spines facing inward, this way reading becomes a true search for enlightenment. ¥