I find that standing up while quilting helps a lot. My cutting table is just the right height, so I place my sewing machine on that. Standing up seems to keep my shoulders relaxed.
There is a Youtube video on how to create a quilting table around your sewing machine (I intend to do this soon):
http://www.youtube.com/watch?v=g14govA4pIM
This also helps with moving your quilt around, whether sitting or standing up to quilt.