I literally have a drummer stool i got on amazon that i sit on. like a 5" thick black stitched pad. attached to caster wheels. it's perfect because I have an antique mahogany desk from the 1930s and the table top is perfect for writing, but too low for most chairs.. :)

the bonus is the back is non existant, so when i get tired i know and its a clear sign to get off of the thing. no crumpling in the chair, when there's no back. :D