I would agree with the Aeron (although, as has already been mentioned, don't buy ANY chair without trying it first--they're NOT one size fits all!), or a Humanscale of some sort: {Link} (you can often find them on eBay for terrific prices--along with the Humanscale keyboard trays which are, bar none, the ULTIMATE in ergonomic joy!)...but I'd also suggest trying a ball.
Esther suggested a Gaiam ball chair--but in trying to decide whether or not to get one of them for myself (since I couldn't find one to try), I bought a simple exercise ball for $10, and tried it out...and was hooked! I've never sat on anything else that was so very enjoyable--and the price was certainly right! ;)
The key to sitting on exercise balls is, that they encourage active sitting. Force it, actually. Which means that you're constantly making tiny movements and adjustments as you sit--all of which strengthens your core muscles, and leaves you much less fatigued, after 8 hours at your desk. Plus, when you're bored, you can wiggle your behind around, and wobble comfortably around the floor a bit--and then finish it off with a bit of bouncing. You can even roll back over it, and let your arms hang over your head, while your spine stretches out. It perks you right up. ;) {Link}
(It is very important to get the right size ball for your height, however, so that your legs are at the correct angle...if you do a bit of searching, some of the places that sell the heavy duty balls will list the correct size for different heights.)
And a ball is inexepensive enough that you can get a new one, and not worry about second-hand anything. ;)
(As a final plus, exercise balls conduct sound in a very cool way...so you can use them to make cool, "alien" noises. You can't say THAT about your average chair! :P)
I would agree with the Aeron (although, as has already been mentioned, don't buy ANY chair without trying it first--they're NOT one size fits all!), or a Humanscale of some sort: {Link} (you can often find them on eBay for terrific prices--along with the Humanscale keyboard trays which are, bar none, the ULTIMATE in ergonomic joy!)...but I'd also suggest trying a ball.
Esther suggested a Gaiam ball chair--but in trying to decide whether or not to get one of them for myself (since I couldn't find one to try), I bought a simple exercise ball for $10, and tried it out...and was hooked! I've never sat on anything else that was so very enjoyable--and the price was certainly right! ;)
The key to sitting on exercise balls is, that they encourage active sitting. Force it, actually. Which means that you're constantly making tiny movements and adjustments as you sit--all of which strengthens your core muscles, and leaves you much less fatigued, after 8 hours at your desk. Plus, when you're bored, you can wiggle your behind around, and wobble comfortably around the floor a bit--and then finish it off with a bit of bouncing. You can even roll back over it, and let your arms hang over your head, while your spine stretches out. It perks you right up. ;) {Link}
(It is very important to get the right size ball for your height, however, so that your legs are at the correct angle...if you do a bit of searching, some of the places that sell the heavy duty balls will list the correct size for different heights.)
And a ball is inexepensive enough that you can get a new one, and not worry about second-hand anything. ;)
(As a final plus, exercise balls conduct sound in a very cool way...so you can use them to make cool, "alien" noises. You can't say THAT about your average chair! :P)