Relax your arms and hands when not typing. Long fingernails force typists to extend their fingers and wrists to hit the keys. This can place a great deal of stress upon the wrists and lead to carpal tunnel or other conditions. Choose a chair with a lumbar support or place a pillow in the small of your back while seated. Also, sit all the way back in your chair to achieve the optimum support.
Many individuals use far too much force when they strike keys or click a mouse.
Keep your head up and situated squarely atop your shoulders. Keep your shoulders back, chest out and feet planted on the ground. Relax your shoulders, use your arm rests to support your elbows. This will allow you to keep your head up, in line with your spine and reduce the stress on your back. Likewise, position the monitor directly in front of you when you type so you do not need to twist your neck to either side.