Ergonomists generally agree that there isn't a single static seated posture that should be used all of the time. It is a good idea to move around into different postures throughout the day to improve circulation and reduce muscle fatigue. If you do sit for long periods, these tips will help reduce strain on your body:

Upper Body

• Make certain that your head is balanced.
  Tilting the head back or too far forward for extended periods will put strain on the neck.
• Upper arms should be close to the body and relaxed.
  Not tensed, out to the side, or flexed forward.
• Wrists should be level with forearms.
  A slight deviation is OK.
• Make sure the armrests don't interfere with arm movements.
  If they do, lower them out of the way.

Lower Body

• Make sure your feet rest comfortably on the floor or a solid surface.
  If you do not have an adjustable chair, make sure to provide a footrest. See Tips for a Less-Than-Ideal Workstation for some ideas.
• Be sure that your feet rest ahead of the knees
  Also, note that the seat cushion isn't compressing the backs of your knees

General

• Recline Slightly
  Reclined postures, where the chair back is at an angle of 100Ì-110Ì, often work best. In this posture, the body is relaxed, upper and lower back are well supported by the chair, and back muscle activity and lumbar disc pressure are low. (Sitting erect or leaning forward increases the strain on the lower back--it's okay for short term use, but it should not be a habit!)