Scorch offered some very good advice. I'm no expert, but I'll add a thought or 2:
Adjust your position so when relaxed, your sights are pretty much dead on. Putting the rifle to your shoulder any old way, placing your support hand any old way, and settling into any old position forces you to use your muscles to put the sights where they need to be. It's very tiring, and not particularly stable.
Are you using a sling? Used correctly, they really do a great job of stabilizing your position without using your muscles. There are a number of good youtube vids by competitive rifle shooters on how to properly set up and use a sling.
Finally, maybe consider aperture sights. They're very accurate for target duty. AR15s with A2/A4 uppers also seem to have a pretty high sight axis, and may be easier on the neck.