Have you tried visiting a chiropractor? (for starters)
I've found that a good one does wonders.
Have you checked the heights of your sewing machine table, your chair, etc.?
Is your lighting decent?
Might there be something else wrong with you?
I, too, get that pain/tightness/ache between my shoulder blades if I try to sew for long periods of time. The simple solution is to vary the tasks - cut for a while, sew for a while, press for a while, pet the cat for a while, find something to eat, etc.
If it's fairly achy, I take a couple of aspirin and that usually takes care of it.