In his book Somatics: Reawakening the Mind's Control of Movement, Flexibility, and Health, author Thomas Hanna offers great insight into body usage with tons of applications for musicians. His work focuses on the habits we develop as a result of three different types of reflexes: the Red Light Reflex, the Green Light Reflex, and the Trauma Reflex.
The Red Light Reflex is the body's response to fear, beginning in the jaw, eyes, and brow and working is way down through the body, ending at the toes. The Green Light Reflex is the "Get up & GO!" reflex (think arching your back). And the trauma reflex is our body's response to injury and it's attempt to guard against pain. His book is pretty fascinating and I definitely recommend it. But today I want to focus on the Red Light Reflex in music making.
As a woodwind player, I often have a furrowed brow when I perform. After reading Hanna's work, I understand that this is the Red Light Reflex in action. Perhaps, I'm working on the clarinet excerpt from Peter & The Wolf and I'm concerned about the cat "falling out of the tree" or nervous about being completely legato as I cross the break in second movement of Brahms Symphony No. 3. Whatever my fear is, it often shows up in my forehead and inevitably starts working its way down the rest of my body.
A simple way of address this is to put a piece of tape on my forward. While I look pretty goofy, the presence of the tape heightens my awareness of any movement in my forehead. Once I can easily feel those movements, it becomes pretty easy to stop furrowing my brow while I play. And as my forehead relaxes, the rest of my body (and my mind!) starts to relax too, and my technique begins to improve. After practicing with the tape for a while, start to increase your awareness of how you are engaging your forehead without the tape and begin to disengage your forehead as you practice & perform.
This is just one application of the book for musicians, but I highly recommend that you check it out for yourself, as he is an authority on the subject and has a ton of great insights. Regardless, I hope this tip helps or your students and I would love to hear what other ways you have found to increase your body usage awareness while performing. Let me know in the comments!