If your job does require you to sit down all day and your boss isn't too keen on the standing desk idea, try to get up every once in a while to stretch out your muscles and loosen your joints. It is recommended that you get up at least once an hour, even if it is just to talk to a co-worker, walk to the toilet, or make yourself a drink.
An even better idea than this is to get up and do some stretches or some light exercise to keep your body active and mobile. Whenever you can, get out of the office. On your lunch break, rather than staying in and seated at your desk the whole time, try to find something to do. If you have extra time after eating, try to give yourself some time to go for a walk. The