Keep the mouse and keyboard within easy reach. Do not overstretch your arms. Keep your arms at a right angle to the shoulders and body.
You would be surprised how many people have an incorrect shoulder position because their keyboard and mouse are incorrectly placed. And being in this position for 8 or more hours per day is a real killer for the wrists, forearms, elbows and shoulders.
Its also a great idea to use a keyboard support matt, and mouse support to rest the wrists on. This takes tension out of the arm and shoulder muscles, and helps reduce the dreaded RSI (repetitive strain injury).