Well, recently, I'd bought a full face Netti helmet. I noticed that the mouth guard rode up whenever I hit anything a little hard, so I got this thing called a chin jock that fits under your helmet and stops the full face from riding up. It's really handy and Im not a sponsor, but im saying that this is one of the greatest things to happen to full face helmets. Its a tad pricy at 30 CAD incl shipping (only pricy for those w/out jobs ) but it's well worth it.
Ps. Duct tape isnt necessary, i just like putting it over stuff