I also had RSI which progressed to the point where I was starting to worry if I'd be able to keep using a keyboard at all. A standing desk had helped for some time, but it was reaching the point where I'd stand for 3/4ths of the day, come home, and simply lie down.
I went to physical therapy, which identified problems with my neck and shoulders, and then we began working on those muscle groups and some of my core. I should note that these exercises were very gradual, such as using no weights or 1-2lb weights -- I didn't go straight to lifting!
After a few months of this, most of my symptoms have disappeared (I only went to phys therapy for about 1.5 months; afterwards I was able to keep doing the exercises at home). Also, my wife has noticed that my posture has naturally improved as a result of the exercises, such as my shoulders no longer rolling inwards.
I still have some remaining long term problems that I've had for a decade, but now I can sit and use a computer the whole day with very few problems. Of course, everybody's situation is different: physical therapy, surgery, and "The Mindbody Prescription" mentioned elsewhere have not helped a friend of mine. But at least in my case, it was a bit humbling to realize that all I really needed to do was exercise (and I'm not overweight, either).
Something else that helped me was getting a quality mechanical keyboard with low actuation and learning how to use it without bottoming out. The model that I like (CM Storm Quickfire Rapid) is unfortunately no longer made, but it uses Cherry MX Brown switches (not loud, but still has an actuation point) and has no numeric keypad, so I can keep the mouse close by without having to stretch my shoulder. I use blue switches at home, which have a more clear actuation point and thus are probably better for a mechanical keyboard novice, but they are probably too loud to use in an open office environment. This guide is very helpful for understanding the how & why of mechanical keyboards: http://www.overclock.net/t/491752/official-mechanical-keyboa...