I keep my gaming laptop on a desk (on a cooling pad) and I sit on a dining chair.
I don't like office chairs, they're not comfortable for me, they're too soft and also I don't want a rotating chair. Padded dining chair is best for me, it's not moving around and it's sturdy even if sit on the edge of it.
I can't play for too long, because of my eye problems, so I usually limit my game play on 1,5-2 hours. If I play longer my eyesight gets blurry and I get a headache. I never get problems with my back even if I'm sitting on a "wrong" kind of chair.