I attended a workshop last weekend in Postural Assessment, with the college of classical massage, Brighton. It contained a lot of information which will prove very useful to my clients.
One basic tip which would help to alleviate much back and shoulder pain when sitting at your desk, is to make sure you are actually sitting on the bones in your bottom, and not rolling back behind them, which an awful lot of people do. If you are sitting on the bones, with your knees lower than your hips, your back automatically straightens, your shoulders go back, and you are not putting strain on any muscles. This position enables you to sit comfortably for a much longer period of time than the usual slumped back, shoulders rolled forward, and neck stuck forward, resulting in much pain and discomfort.