Try making a conscious effort not to toss and turn right when you feel like you're about to sleep. I have to do that sometimes or I end up on my stomach (i.e. "flat"), which means my head is turned to the extreme side (parallel to my shoulder) and I wake up not being able to turn my head in the opposite direction. :/

Thankfully, since I started making a cognizant effort when I feel very tossy-turny, I can honestly say that the neck problem hasn't happened since. MAYBE THERE IS A CORRELATION but who really knows.