If you put a rolled up sock in your mouth at night then I can assure you that you wont become a hunchback.
Or roll up the sock and shove it between your shoulder blades. Then lie back and mouth breathe all you like. The sock will counteract the hunching.
Seriously though, I suggest a few yoga postures when you get up. Or just before you jump into bed - may make a world of difference. In yoga breathing and posture are entwined. So it may help to reverse some of the muscle tightening that happens when you are asleep.
Borrow a book from the library (we forumites love a freebie!) and look up a few postures that are good for neck/back/shoulders
Yoga is top stuff. My new year's res was to do yoga twice a week. Partly to make myself meditate regularly. After a few weeks I found it made such a difference I have been doing it pretty much daily.