I use a secretarial chair as well, adjustable up and down. Seems to work well for me. My sisters sews while sitting in one of those ball chairs - looks like an exercise ball, but the ball sits in a frame with wheels. Forces you to sit straight and use your core muscles. I haven't tried it out, but my sister claims it's the best. My brother also uses this type of chair for fly tying and for playing his guitar and mandolin. He's had lower back surgery and claims this chair and saved him. Check it out:
http://www.amazon.com/Gaiam-Balance-.../dp/B006JBWYDA