searching for a computer chair

My old "chair" is more than a decade old.  It's served me very well, but it's time to get another.  Does anyone have suggestions for devices (chair or otherwise) that encourage good posture even during long bouts at the computer screen?  It also needs to be cheap, which cuts into the options pretty quickly.

Has anyone used a giant rubber ball as a fulltime seat?  I've tried them just for a few minutes at a time but they didn't quite seem comfortable to me.

Are there chairs that support split keyboards, one at each armrest?  That seems like it'd be ideal, but that's probably expensive because it's more of a medical device to help avoid aggravating carpel tunnel syndrome.

My current chair was one of those kneeling shin rest sort of contraptions.  I've enjoyed it a lot, but I think I'd like to try something different for the next decade.  :)

Any suggestions?