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).