For those of us spending much of our work day on a computer, taking care of our bodies at work is a priority. Not only does it help the employee feel better during long days at the office, but companies should also be concerned as employees are increasingly taking time off work and even applying for worker's comp because of injuries sustained at a desk. Repetitive Stress Injury, or RSI, is one of many complications that can arise from the combination of poor posture and all-day use of tools that are not optimized for a human user.
A few products and some tips for sitting and using your computer should reduce discomfort and the possibility of work-related fatigue or injury:

Head/Neck & Eyes:
Place the top of the monitor at or below eye level - never position it higher so that you have to lean your head back.
Position your monitor approximately 20-40 inches from your face. Use a larger font if you can't read from your screen at this distance.
Low lighting can help you avoid glare, which can cause eye strain and headaches over time. If you cannot avoid glare, use a glare filter to reduce the light being reflected into your eyes from the monitor.

Back:
Sit with your head straight between your shoulders. Do not lean or tilt your head. If the position of your desk or monitor forces your head to an awkward position over your shoulders, you should try to move it so that you can sit straight.
Your knees should be either straight in front of your hips or slightly lower.
Place your feet at an angle to the ground using a foot rest. Having your feet at an angle will relieve stress on your lower back.
Invest in an ergonomic chair or a backrest at the very least. A chair will support your body in the correct position and most are adjustable so that you can customize according to your body type. A backrest will help by supporting your back and putting your body in the correct sitting position.

Wrists & Arms:
An ergonomic mouse and keyboard are essential. They are relatively inexpensive, so there is no excuse for anyone to be using a mouse or keyboard that isn't designed for comfort.
When choosing a mouse, remember that no matter how comfortable it is to use, it is important to vary your position and movements periodically throughout the day. Repetitive movements and long periods in the same position put you at risk for strain and injury.
Maintain a loose, comfortable grip on your mouse and take breaks to stretch and rest.
Move with your elbow to move the mouse rather than resting your elbow and moving with your wrists.

A good overall tip is to remember to stand up, walk around, and stretch throughout the day. This will keep circulation going and relieve pressure in body parts that you may not even notice you are stressing.

If you are really interested in the best position for your body, an ergonomic workstation is the best investment. These systems cradle your entire body and position your chair, monitor, keyboard, and mouse in the ideal positions for zero stress on your body.