Sitting all day long at work is not the best way to stay healthy. Many people are using standing desks to stay more active at work. Investing in a decent active sitting chair is not a bad idea either. The Sit-Fit chair might be just what you need. It uses the concept of instability to engage your core and promote better posture.
The chair has a pneumatic lift that allows the height to be adjusted easily. This project is currently getting funded on Kickstarter.