Try using a footrest, perhaps a rocking one so that your feet get to move a bit while working. It should help you maintain a better posture.
You should also try to get out of your chair and move about for a few minutes as often as possible throughout your day. Among other things, the slouching/hunching/shifting in your chair is often an attempt to alleviate the pressures placed on your spine while sitting. Forcing myself to get up with a timer made a huge difference with my back pain. Before, I was at my chiropractor's office at least once a week.
You should also try to get out of your chair and move about for a few minutes as often as possible throughout your day. Among other things, the slouching/hunching/shifting in your chair is often an attempt to alleviate the pressures placed on your spine while sitting. Forcing myself to get up with a timer made a huge difference with my back pain. Before, I was at my chiropractor's office at least once a week.