I'm typing from an Ikea Marcus which is my home office chair.
While I agree that the Aeron is great (and have one at work that I use regularly), the Marcus fit in my budget ($200 for the Marcus vs $800+ for a new Aeron) and is perfectly reasonable. I usually leave it in the locked forward typing position.
For twelve hour stints, I stand for part of the time (my desk will move up to standing height).
I tried an exercise ball - didn't work for me.