It's probably apparent by now that, in the final analysis, players find what's comfortable and right for them through experimentaton and a natural evolution of playing technique that happens over time. Truthfully, it is unique to each individual. In the early stages of learning to play, the mid-back muscle tension (between the shoulder blades) and tension in the rotator cuff of the right shoulder is very common and, peoples' playing style -- everything from where they stand, to how they create vibrato, to where they position their arms, and the height of theremin in relation to the body -- is impacted so greatly that many times, the playing technique unconsciously adapts primarily to minimize the discomfort. It can really affect one's ability to be expressive.
When I teach others, I help them to find their settings and techniques by having them do the following -- try this for just fifteen minutes each time you practice. I refer to this as "getting out of your own way." Consciously, we try to do what's correct, but unconsciously YOUR BODY ALREADY KNOWS. Sounds a little heady, but this works with every student I've had, regardless of age or experience.
a) You've seen lots of people play by now and tried a lot of things. For fifteen minutes, FORGET EVERYTHING.
b) Stand where you normally stand and close your eyes, hands at your sides. Take ten very deep and slow breaths.
c) Keep your eyes closed. Remain unconcerned about PLAYING MUSIC. Once you've completed your tenth breath, exhale completely. Now you're ready...
d) As you inhale again, allow your arms to float up in any way THEY want to and just let them float around as the theremin begins to respond any way IT wants to. Allow the theremin to make sound ONLY as you inhale. Now just breathe deeply as the sound plays and your arms move, become acutely aware of where the points of tension are, where things feel relaxed and where things feel strained. As you continue to breathe, relax all the spots you felt tension in WHILE YOU PLAY. When you feel that tension release, THEN exhale, allowing your arms to float back to your sides. Do this for at least five more slow, deep breaths. Eyes closed the entire time.
e) Think of any melody or song you absolutely love -- one that carries all the joy and passion that no other song seems to have. Hear it in your head as fully as you can.
f) Know this: whatever energy or combination of energies out there in the universe that is required to play a theremin -- it's always out there. Move out of your own way and begin to allow a shift in your thinking. Allow whatever the universe requires to flow through you. Stop thinking of yourself as the solid entity that bends this instrument to your will -- rather, think of yourself as the conduit and ALLOW THE THEREMIN TO PLAY YOU -- and still with your eyes closed, play that song you love. It doesn't matter if you get all the notes right or the speed -- JUST PLAY. Your body knows what's comfortable. Let it find that place for you.
I promise you, odd as it may sound, this mindset will do more to help you as a player than you can imagine. Your body will arrive at techniques you would never have conceived of if you'd kept your eyes open. We are so visually oriented that we look so hard we disallow the intuition.
Try this for a few days and see what happens.