I like height-adjustable desks, but I also think there can be great benefit in “alternative” chairs if you can find one that suits you (e.g. saddle chairs, kneeling chairs, bouncy chair-on-a-stick chairs, etc.; personally I have a HÅG Capisco chair, which is amazing for me). If you can, try to find a place which has some of these chairs that you can try for at least a half hour or so at a time, and ideally see if you can borrow one for a few days to see if you like it before buying anything. This place in San Francisco will let you sit and work all day in their showroom if you want to try their chairs:
http://www.ergodepot.com/category_s/168.htmIf you’re on a budget, this thing perched on top of any standard chair is pretty nice:
http://www.ergodepot.com/HumanTool_Balance_Seat_p/ht-balance.htmFor me personally, standard office chairs, including the $1000 type (Aeron, Steelcase, etc.) are uncomfortable for sitting in more than a few hours per day. I prefer a simple hard wooden chair, like a kitchen chair or whatever, over the kind of chairs in most offices. But changing positions frequently, e.g. sitting on the couch for a while, lying on my side for a while, standing for a while, etc. is very helpful, as is taking frequent breaks to get up and walk around.