I think you need to do core strengthening exercises in order to correct poor posture, because you have to develop the muscles that help you hold yourself upright and those muscles are in your abdomen and lower back. I'm not a fitness expert, that is just what I have heard. I'm sure there are specific yoga postures that help your core, or there's always stomach crunches and planking! It takes awhile to really develop those muscles, though, so you might want something to help in the meantime. I know people who do a lot of heavy lifting wear a type of brace that supports their lower back. It's not the most glamorous looking thing, but it would probably help. Something slightly less conspicuous (because you wear it under your clothing) is a "waist-nipper" that has flexible boning in it (a full-on corset would probably be even better, but a good one would be expensive and not many people would want to wear such a thing)--one made of strong stretchy fabric and has hooks and eyes. You wouldn't want it to be really tight since you aren't actually using it to make your waist smaller (unless you want to, in which case go for it), but the flexible boning in it would help you stay upright (because being bent over would be more uncomfortable!).
If your wrist hurts you at all, you might want to look into getting a wrist brace from the drugstore. You don't want to get carpal tunnel!