A good posture is one which most naturally suits your body. This means that your back should be in a slight S-shape with your shoulders up and your head straight. You should not be hunched over nor have your head tilting down at a sharp angle.
Your arms and fingertips should be horizontal and level with your desk top.
The back rest of your chair should support your lower back and you should sit with your back against it.
Your eyes should point slightly downwards to the middle of your computer screen. Adjust the height of the screen accordingly.
Your legs should be horizontal on the seat of your chair, or should point slightly downwards at the knees to encourage a good posture (try tucking your legs under your chair).
If your feet do not touch the floor, a foot rest should be used to support your legs.
Try to become more aware of your posture and keep reminding yourself to sit properly. You will soon learn to do it naturally and you will benefit forever.
Changing your body position regularly keeps your back, neck and body mobile and free from stiffness