In terms of something to work towards, a Coach I spoke to said that the exercise he uses to help correct "office" posture is the overhead squat. It's a pretty advanced exercise and you'll have to go super light to start - as in use a Broomstick first, but it's something to work towards. You'll definitely find out which bits of you aren't flexible the first time you try it, especially back and shoulders.
http://www.exrx.net/WeightExercises/OlympicLifts/OverheadSquat.htmlObviously do these in combination with a balanced training routine.
Something I do at work is put something small between your mid/upper back and the chair and keep it lightly pressed there, if you slump forward, it drops.