Have a Well Designed Chair: If sitting all day is unavoidable, then having a good chair to do it in is a must. Look for a chair that has firm cushioning and easy height adjustments. Lower back and well-placed arm support are mandatory.
Good Posture: This seems obvious, but the best chair will not do any good if the person sitting in it is slouched over all day. The spine should be straight, keeping the torso perpendicular to the floor. Arms should come straight down from the shoulders, not forward or out, and forearms should extend straight forward at the elbow. Joints should be at ninety-degree angles.
Mouse, Keyboard and Monitor Positioning: The keyboard should be front and center of the chair and near the edge of the desk so that the arms can stay in the position described above. The mouse should be as close as functionally possible to the side of the keyboard so that reaching is not necessary. The monitor should be directly behind the keyboard and just below eye level so that maintaining a neutral spine is possible while viewing.
Move Around: Sitting all day, even in the best of conditions, is not healthy. Try to stand and walk around as often as possible, even if for only a few seconds.