Here's a motivating thought -- your back is countered by your stomach -- when you walk, do cardio, etc. - if you focus on keeping your abs tight, then you notice that you tend pull your chest up , shoulders out, and voila - your back straightens up. There's no feeling i love better than sore abs after a good workout - so just focus on that.
I noticed my posture improved massively as I went into comp prep -- I'm a 15 year computer jockey so I know where you're coming from. But I was amazed at how good it felt to keep everything upright!