I have my sewing machine sitting on a piece of carpet and I've shoved 2 rubber door stops under the back of my machine so I can easily see my needle. This allows me to sit up straighter and eliminates upper back pain.
I felt like I had developed shin splints after piecing basically non-stop for 6 weeks (probably only 14-16 hours a day, with breaks to iron) and switched my pedal foot to fix that. Guess I got lucky that my back didn't hurt first!