Also, it is important to move around occaionally... not a bad idea to stand and stretch every 15-20 minutes or so... Just stretch the areas of your body that have been flexed... neck forward? pull it back? shoulders hunched? go to the doorway and, holding onto the door jamb on either side, lean through the door, stretching your chest muscles. Hold stretches for at least 15-20 seconds. Don't stretch til it hurst... stretch til it feels good! Then hold that position for 15-20 seconds, minimum... Don't forget your hips... Stand up, keeping knee straight, slide foot behind you until the front of your hip is stretched...
Great book: "Stretching" by Bob Anderson
All these suggestions about the correct height for table and chair are very good!!