I like to sit in my big comfy office chair and produce (Big comfortable and supportive office chair with wheels is a must) that lets me completely go off into my creative space.
I also do graphic design for my business on my computer so it's a double whammy for me. I'm usually on the thing for 12 hours a day.

Damn computers can do so much fun shit!

The way I have combated the sore bum factor is by getting up in the middle of it all and go for a bike ride or do some kind of intense physical activity and that helps to jump start the creative flow again.