How many of you have seen a guitarist make faces while performing? (How many of you make faces while performing?)
We guitarists are not alone in this (cellists tend to mug on stage as well). But I've seen it enough that I recognize it as a common problem. Further, in working with students to try and eliminate the problem, I recognize it as especially difficult to eliminate, once established. I have a simple excersise to fight this problem that I'll suggest below.
First, it's worth noting that facial expressions are a natural and normal part of playing music. We express emotion with our faces (among other things); we feel emotion while playing. So we express what we feel with our faces. This is both fine and good. It's when odd faces enter the picture that it becomes an issue. I've seen players roll their togues around from one cheek to the other, looking as if they were chewing a golf ball; I've seen players roll their tongues over their front teeth as if excavating for popcorn pieces; I've seen faces turned into masks of discomfort even when the music is easy.
It's worthwhile remembering that we are in an age of constant photography and videography. YouTube reigns supreme. How you look when you play matters. The audience, after all, comes not just to hear you but to see you. This is important.
So what to do? My remedy for this is so simple it seems silly. The excessive facial movements some players make are subconscious; they aren't doing it on purpose, it just happens. My approach is to simply remind them of their faces. To do this, hold a small object between the lips while playing. I like a Bic pen cap, but anything light will do. It feels odd hanging there. And so reminds the player to channel the energy away from the face and towards the music in more productive ways.