Be ergonomic by making sure your work area is a good fit for your body. How you sit where your desk is, how you set up your monitor and/or workstation can make a big difference in how you feel at the end of the day.
If you are having pain in your arm try a trackball mouse. I won't compute without one anymore. And if you feel a little depressed, try opening the window shade or installing natural lighting. These little changes can make a big difference in how well you do you work and how long.
Are you working ergonomically?