I wrote this as a comment to the LJ group Knitting that had a post
regarding neck and shoulder problems and whether anyone had tips. This person specifically has fibromyalgia (which I also have), but if you don't spend a lot of time knitting, it can really hurt if you try to knit a whole day's worth of knitting.
For your shoulders, you can do door way stretches which not only stretch your neck and shoulder muscles but roll your shoulders back where they should be. Stand even with a door frame. Put your hand on the door frame (higher or lower than your shoulder, depends on your stretching needs). Make sure your hand is rotated so your thumb is straight UP towards the ceiling. Make sure your back and neck are straight. Take a small step forward till you feel a gentle stretch. Swivel your head over the opposite shoulder, continue the rotation with your eyes looking over the opposite shoulder. Again, a gentle stretch. As you know with FM, too much of a good stretch will make you flare.
For your neck, here are some proper body mechanics tips. When you knit, and when you do the shoulder stretches above, make sure that your chin is tucked back and your head floats upward. To get there, push your finger on your chin so that your head slides back (like a turtle pulling its head into a shell) and your neck feels straighter (removing the forward head carriage we get from fixating on knitting and working on computers). Then imagine a string is attached at the top of your head and it floats upward. This will remove the double chin effect from turtling your head back. Take a deep breath and roll your shoulders back. Tilt your sternum on your chest upwards (as if you had a batman logo on it and your very proud of it).
I've been knitting for 4 years, and every year I seem to be able to knit for longer periods of time. I think over time your hands, forearms and shoulders become stronger. If my neck is really tight, I will I often put a pillow on my lap and rest my arms. This puts the knitting closer to my face reducing me from pulling my head forward. Also the muscles in your neck and shoulder will relax since your arms are being supported not hanging out in the air like a T-rex.
I am not a doctor, but I've had A LOT of work done for my FM which turns out was undiagnosed Lyme Disease for 13 years. I am not responsible if you hurt yourself and other legal mumbo jumbo stuff applies. I hope all this info helps. It's good for anyone with neck and shoulder issues often due to sitting hunched over.