I use a swiss ball at home, but i find this cushion does enough for my back, and posture while in a computer chair:

Looks kinda like a whoopee cushion, but less obvious than a swiss ball in the office

http://www.fitball.com/catalog/balan.../dis-o-sit.cfm

http://www.performbetter.com/catalog...H4X86U0GH32B95

check it out

FXjohn