The computer serves you, not the other way around.
Sit so that the monitor and keyboard are directly in front of you.
Make sure that the monitor is not too close.
You should not have to stretch to use the keyboard or twist your torso to any degree.
When you feel bored, tired, restless, stiff or have sore eyes - get up and take a break.
If your employer will not allow you to get up, find out your rights.
The more tired you get, the more you neglect your body and poor posture will only tire you further.
Do not ignore your body; you need it beyond the office.
