All you need to improve your sitting posture is a Windows PC and a Kinect.
With SitPerfect running dangerous music starts playing when then angle between your shoulders and your head is less than 55°.
This is by no means scientific (yet).
If you want to try and have some fun: jmechtel@gmail.com
To stay motivated I wanted to start measuring my posture over time. Now SitPerfect writes a logfile with angles between my head, neck and shoulder. Then I can create simple charts like this one and hopeful see where my efforts to improve posture are taking me:
Download and install Microsoft Kinect SDK. This will work with the Xbox 360 USB Kinect. For the dedicated PC Kinect, the runtime from the SDK would be enough.