So, the other day I was practicing and felt my left shoulder wanting to hack itself off my body and start a life of its own.
I thought, 'Why does my shoulder absolutely kill when I'm practicing?!?!' and found part of the reason.
I do much of my practicing while sitting down, but with a strap.
I realized the strap was firmly resting on my left shoulder.
I looked down and realized that was because the guitar was slowly creeping forward on my leg while I was playing, putting stress on my strap, and, thus, my shoulder.
Having a non-slip pad on the waist of the guitar would certainly stop that from happening.
The Gibson Flying V and Steinberger rubber pads popped into my head, and I didn't see anyone selling them at a price that made it feasible for the 15 guitars I have.
A little searching around lead me to the Soft-Touch Self Stick Pads 4" x 1".
I bought a few sets and they worked for what I needed them to- my strap is generally pretty loose on my shoulder now.
I place them right in the waist of the guitar, which keeps the guitar from creeping forward.
The other benefit I found was that I can keep my technique the same as I practice- as the guitar crept forward, I was naturally changing the angle of my picking and fretting hands to compensate, something I never thought of.
Now they're on all of my guitars.
I cannot guarantee that they won't discolor or affect the guitar's finish (especially with nitro finishes), but, on my personal guitars, the reward outweighs the risk.