I don't even have a chair.
I had tried a Hermann-Miller Aeron chair ... like Harriet Hargrave uses but it didn't fit my body. While at the office furniture store I saw what their people used: a Swopper. It is adjustable and like sewing on a balance ball. It keeps me in good posture. I put a throw rug under it to protect the floor as it doesn't have rolling wheels. You can see it on www.sit4less it was well worth the price. I think I've had it over 10 years now. When my friends come to sew with me they love to use it too.
ali