Move

 I am not talking about running a marathon in your lunch hour, just getting up and moving as much as possible. After half an hour in the same position the soft tissues around joints begin to stretch to a point where it will take longer to return to their normal length than it took to stretch them out. If you get up and move around you minimise this stretch out and getting back to normal is easier. 

Move while sitting at your desk by regularly lifting your chest up and tucking your chin in. Get up and move, stand tall, stomach in and squeeze the shoulder blades back and down a few times. These are simple daily changes that you can make yourself and can improve your posture, make you feel taller but also help prevent back neck and shoulder pain that comes with the sustained postures.