Though I am new to the guitar, I have a fair amount of experience in strength training and movement efficiency. We live in a world of cubicles, computers, hand held electronic devices, etc. Such things tend to cause hunched shoulders (kyphosis), forward head (and associated neck pain, etc.
I'd like to recommend that you search for "Donnie Thompson Casual Bowtie" I use it to demand proper form when I do squats & deadlifts. I also began wearing it when I practice guitar (McDonald's "Conservatory Tutor, still on right hand exercises early in vol 1), and it has been a F'n# PITA ...errr... benefit to my form as well.