I've found that the most important thing in decorating an office is making it feel homey. I have filled mine with picture frames, inspirational quotes, and my favorite scent. I also brought in my own lamp, and keep it turned on whenever I'm in the office. This is a place where I spend eight hours a day, everyday. I wanted it to both represent and inspire me.
Tip: If your office inspires you, it will probably inspire other people you work with, too.