I agree with those that posted about putting your machine on a tilt. I didn't have doorstops & definitely wasn't willing to "invest" in a manufactured tilt for my machine, so I just put a roll of masking tape under the back edge & that's been working fine for me. Helps a ton with neck pain. I also took empty chocolate boxes & taped them around my machine to make a platform & then stacked books under my folding table to put next to the old desk that I sew on. Having that extra space to rest my forearms was something that my pain specialist recommended & it has also helped a lot. Of course, I have to put shims under the back edge of the boxes so they match the tilt of my machine.
Also, make sure that the front of your chair is tilted down. If you can't make that adjustment, then either invest in one of those butt cushions that has the slots angled down for your legs, or else take one pillow & put it flat on your chair & take a second one & put it at an L-angle at the back of the chair (so it cushions your tailbone & forces your legs to angle down). I also have found that making an angled support for my left foot helps to balance me out, but I just saw a video with Leah Day that recommends putting a phone book under your right foot, behind the speed pedal so I'm interested in trying that.
Beyond that, I love the 8-hour stay-cold ice packs that you can find at Walgreen's or Wal-Mart and such by the pharmacy (heat only works for muscle pain; ice is for nerve pain/joint pain/inflammation, which are more often the culprits with repetitive motion). They come in a velcro pouch so you can strap them on & keep sewing. If it truly is muscle pain, then I use the ThermaCare heat wraps. Any kind of exercises that work your shoulders & traps will help with neck pain. Core work helps with back pain (I've even seen a few people manage to successfully sit and sew on top of those balance balls -- that is a skill in itself if you ask me! lol). And listen to your body. Don't keep working through the pain! We all stop the moment our machine starts moaning & groaning but will ignore our bodies when they do the same thing. Stop. Stretch. Reposition yourself. Check to make sure you haven't lost proper posture as you've become more fatigued. Switch to another task for a bit -- stand and cut fabric or sit in bed and work on applique or binding another quilt. If it's become darker, turn on some more lamps -- or buy extra lighting so you're not straining forward to see your stitches.
After 2 neck surgeries & 5 back surgeries and countless injections, pills, acupuncture, PT & chiropractic treatments, I completely sympathize with you and really hope you can find some options that work for you!