While some people opt for a fancy, adjustable-every-which-way Aeron chair, designed by Herman Miller, they are very pricey and not-so accessible for most people.
But two other great options exist, things that are sort of outside the usual "chair" mold—ones that can actually help improve your posture and your muscles while you get work done!
1. You can try sitting on an inflatable exercise ball, such as a stability ball from Perform Better which forces you to engage your core muscles and pay attention to your posture while you sit—thereby giving you a subtle workout while you work.
The key, of course, is making sure that you chose a ball that is right for your height. If you are between 4'11" and 5'4", get one that's 55cm in diameter. People who are 5'5" to 5'11" should opt for the 65cm. People over 6 feet will want a 75cm one.
I actually had one of these ask my desk chair for several months about two years ago and it was great! In all honesty, the only reason I stopped using it was that I got a wee bit lazy; when I really needed to get down to business and work fast-and-hard, the ball just distracted me too much. So I started swapping it in and out for my chair—until ultimately I just went back to the chair. The other tricky aspect, of course, is that you need your desk height to jive well with the height of the ball—you don't want to be too high or too low when typing at a computer. See here for more on the proper way to set up an ergonomic work station—with everything at the proper angle and height.
2. Another great option that I tried about five years ago is a kneeling chair. This one I sort of inherited from an old colleague when she moved on to another job, and so I gave it a try. While at first it's just downright odd to sit with your knees on a cushioned platform, I came to totally love it…my back was straighter, my shoulders down and loose, and I felt like I wasn't tempted to slouch or do other bad habits. In the end I only stopped using this chair because I myself moved on to another job and couldn't very well abscond with the chair! (It was office property.)
And hey, if you're feeling VERY adventuresome and determined burn extra calories while at work, perhaps you can go totally chair-less and invest in a Treadmill Desk!!
What desk chair do you use? Have you ever tried using a ball instead of a traditional seat?