We are so often made aware of how bad slouching is but sitting in the perfect position for too long can be almost as detrimental. Sitting up too straight and still for long periods can also be a strain on your back.
Take breaks a few times a day. Make sure to stand up, stretch, and take a quick walk several times a day to keep yourself from getting stiff or causing injury.
Make sure your chair is at a height where your knees are at a 90-degree angle, your feet can rest flat on the floor, and you have proper lower back support. Try standing for part of the day, perhaps while you're on the phone or reading.