UCLA ergonomics is your friend:

http://ergonomics.ucla.edu/office_Chair.html

http://ergonomics.ucla.edu/ws_checklist.html

I dont think there's a perfect chair, just pick a sturdy one that adjusts a lot and keep adjusting it all day. Also, i find that when the chair is on a carpet floor, thus not able to roll easily, i feel more comfortable than when it rolls easily.