Have you considered a chair that supports your posture? Or try rolling a towel and placing in along the mid-line of the spine to open your heart and drop your shoulders back. This helps posture quite a lot. To practice, you can start laying on your back with the towel along the spine. Just relax and breathe there for 10-15 breaths, and then try sitting.
Let me know if you like it...I've got a lot of tricks up my sleeve!