The cushion they advertise on TV as the Forever cushion (I think that is what it's called) has the gel inside and it is really worth the $20 when you find it at stores. Kmart here carrys it for $19.99. A girl at work bought one and wow does it make you feel comfy.
For my office I actually bought one of those extra large exercise balls to sit on. I blew it up as big as I could get it. Everyone thought I was nutz! I don't use it all day or everyday but it makes a big difference. As I sit on it I can rock back and forth or side to side and get some exercise in the core muscles. and when you get tired or frustrated, you can just bounce up and down like a little kid and that makes everything better!
Not sure if it would work in a sewing room or not....