Ten tips to help you avoid fatigue:

1. Make sure that the weight of your arms is supported at all times. If your arms are not supported, the muscles of your neck and shoulders will be crying by the end of the day.
2. Watch your head position, and try to keep the weight of your head directly above its base of support (neck). Don’t “crane” your head and neck forward.
3. Don’t be a slouch! Slouching puts more pressure on the discs and vertebrae of your back. Use the lumbar support of your chair and avoid sitting in a way that places body weight more on one than on the other. Move your chair as close to your work as possible to avoid leaning and reaching. Make sure to “scoot” your chair in every time you sit down.
4. The monitor should be placed directly in front of you, with the top no higher than eye level. The keyboard should be directly in front of the monitor so you don’t have to frequently turn your head and neck.
5. Talking on the phone with the phone receiver jammed between the neck and ear is really bad practice. You know that’s true, so don’t do it!
6. The keyboard and the mouse should close enough to prevent excessive reaching which strains the shoulders and arms.
7. Avoid eye strain by making sure that your monitor is not too close, it should be at least an arm’s length away.
8. Take steps to control screen glare, and make sure that the monitor is not placed in front of a window or a bright background.
9. You can rest your eyes periodically for several seconds by looking at objects at a distance to give your eyes a break.
10. The feet should not be dangling when you are seated. If your feet don’t comfortably reach the floor or there is pressure on the backs of your legs, use a footrest or lower the keyboard and chair.

Source: http://ergo-plus.com/office-ergonomics-10-tips-to-help-you-avoid-fatigue/