one of these:
http://imgur.com/f5fM6.jpg[IMG]
Herman Miller Aeron, been using this for 3 years now, best money I ever spent. Designed specifically with ergonomics in mind.
IMO, don't buy a cheap £75 viking chair, because not long after you'll start finding it uncomfortable and will want to replace it. Do that a few times and you will have spent the same money as buying an Aeron.[/QUOTE]
Our office is decked out with these chairs, a good 40+ on our floor alone. They are awesome, but I would never buy one for home use due to their price.