I sit on an inflated disc cushion in my chair. As others have said, change your position frequently. Move your head, neck and shoulders through different motions. Also, after you are finished sewing, a warm shower is a good way to loosen those muscles up. Here's the link for this product:
http://www.flaghouse.com/Seating%2DD...D15-item-37851
You can probably find it at another site for less. Good luck!