I just had a custom mouth guard made by my dentist. I have numerous cracks and chips on quite a few teeth from gritting my teeth while lifting.
Get a custom mouth guard made. They take a little getting used to as they alter your breathing, but they'll save you thousands in restorative procedures years down the road.
Just grunt and scream like a pig the last grueling reps. As long as I see that people training their ass off I don't mind they make animal sounds occaisionally...
Years ago, I went to the gym ONCE with this man that I was dating. He was a former football player, so I think he got his gym 'etiquette' from those days of showing off in front of his teammates. Man, he grunted, groaned and made awful faces. Then at the end of the exercise, he THREW the weights on the floor. He wasn't doing all this because the weight was so challenging or uncontrollable (if he was too fatigued to set the weight down, he would have dropped it and not THROWN it) -- he just wanted all eyes on him...
Holy crap...I was wondering if I was the only one with this problem. The use of thermos like E/C/Y, etc exacerbates this effect ever more. I'll grind my teeth on thermos even when not lifting.
My brother does this teeth pressing thing but i have just the opposite behavour. I allow my mouth to hang open the whole time and i groan rather a lot. Rather unnattractive i belive. Wish i could stop this habit. I would rather to have the teeth problem and use the guard to protect them.