I just sent this as a PM to be sure it was received, and am posting this in the open forum in hopes that a LOT of people will see this.
Our quilt guilds (I'm a member of 3) and sewing guild (just 1!)have had four different physical therapists give hour-long programs in the last two years specifically about the biomechanics of sewing/quilting, how to select proper equipment, how to avoid repetitive motion injuries, and the like.
Every one of them has said first thing: DO NOT GO CHEAP ON YOUR CHAIR: SPEND THOSE FEW HUNDRED DOLLARS (between $350 and $500) ON A CHAIR THAT WILL PROTECT YOU, BIOMECHAICALLY SUPPORT YOU PROPERLY AND (thus) ENABLE YOU TO DO WHAT YOU LOVE TO DO.
A well-selected, top quality chair ALONE saves you hours/days/weeks lost due to back, shoulder, etc. discomfort/pain/stress/strain. If you already have such discomfort, an excellent chair will decrease the discomfort noticeable. If you DON'T have such discomfort, don't set yourself up for it, don't set yourself up for physical therapy down the road.
Tweak the chair's adjustments so that when you sit in it, your spine is straight and you automatically realign yourself properly when you sit in it. After I "dipped" the front of my chair down a tad, I felt so much relief and can sew for hours.
I have a friend who just bought the GYPSY SIT UPON and she loves it - it works the same way as the HUGE balls some people use instead of chairs or in exertcise classes: it realigns the spine, strengthens the lower back and upper back muscles and does a lot to keep your shoulders from hunching while sitting over what you're working on.
My mom (85 years old) uses a top quality memory foam wedge-shaped seat cushion. She has tailbone problems and hates the "doughnut" cushions - but finds that this works well. That said, my sister and I dropped $500 on her chair and another $100 for the cushion (approved by my orthopedic surgeon-brother) ordered through a physical therapy store.
Hope this helps.