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.

If in an established company, ask for a new chair. For decades I put up with whatever POS I could scrounge ... until I discovered that one could just ask for a new chair, and whoever was tasked with corporate ergonomics made sure I had one pronto. Maybe not perfect, but chosen for reasons known by someone tasked with understanding such things, and way the heck better than any discarded seating we're used to.

Most reputable and higher end stores will allow you to take chairs home and try them for several days at a time. Do not make the decision on what chair feels the most comfortable to sit in at a store.

If you've been sitting in cheaper chairs and have bad posture then immediately changing to a chair which forces you to site correctly is going to feel uncomfortable for awhile and you might even experience more pain initially.

It's very important to try any given chair for at least several days before purchasing (even at that, sometimes days is not enough).