I have two tips.
One is yoga, and in particular the "cobra pose", to flex your back in the opposite direction. You can do it several times a day, also at work.
The other is to increase awareness of your posture. Download a time-lapse video app for your phone, and make a time-lapse movie of yourself sitting at your desk during a day. You can show this movie to a therapist, so they can comment on it.