For me, the best antidote to sitting, whether at a desk or in a car, is walking. I find the gentle motion it seems to encourage in the spine will help right any wrong posture, although my personal approach is to take the beginning of a walk and really try to straighten up and loosen up in various ways until it starts to feel normal and natural again - right after you get out of your chair, it won't, and if you don't go looking for better posture during a walk, I'm not sure everyone will just eventually find it; I can certainly walk with crappy posture for a long time if I'm not paying attention.
-S-