OSHA guidelines are helpful in figuring out how to position your workspace so it is ergonomic. Here is their website:
http://www.osha.gov/SLTC/etools/comp...positions.html
Although it is for computer workstations, it's very applicable to a sewing workstation also.
I used to get that pain between my shoulders from quilting on my domestic machine while sitting down. With experimentation I found that I do not get that pain when I quilt standing up. (This is for quilting, not piecing. I still piece sitting down.)
To quilt, I place my machine on my cutting table (which is at about belly button height) and place a styrofoam table around my machine to provide a flat surface to support the quilt. There is a Youtube video on how to make this table at:
http://www.youtube.com/watch?v=g14govA4pIM
http://www.youtube.com/watch?v=yAS25v3ZTk0