Found that I tend to slump down in a chair with a back. So, I just use a stool that's low enough, personally. Or an ottoman. Works quite well, in that it reminds me to have a better posture when sitting, and, if the seat's small enough, I can just plop down on it and let the pleats hang over the back.

Been using a leather ottoman as my computer stool for the past year... Works great.

-J