Go into a Staples or an Office Max or an Office Depot, and start sitting down in a couple of them. Some of them are just plain more comfortable than others, and it was something I noticed right away. I would tell you what I decided on, but I don't remember.
A couple things you should look at: adjustable height (I actually prefer the ones that aren't hydraulic, because I tend to sit at the same height all the time); make sure there isn't a slight rise right at mid-thigh (cuts off your circulation); also, make sure it doesn't press on other pressure points; good lumbar support (everyone has their own, so adjustable is good, but if you find one you like that isn't adjustable, that's fine, too.
Go towards the end of the day, when you're already tired.