Upload & Sell: Off
I've always gone for chairs from manufacturers known to produce ergonomic stuff -- Herman Miller, Humanscale, knoll and some higher-end Steelcase. Having said that, I'm a cheap b*stard and would never pay full price for a fancy chair, which is why I'm currently sitting on a fully-loaded Aeron that I bought used on Craigslist for $300
I also have a pneumatic height-adjustable stool I bought at an art store that's great for shorter stints. The stool forces me to sit upright and stay alert. The Aeron encourages me to relax and doze off.