Playing shakuhachi involves awareness of your heart/mind, your body, and your breath. When you play and begin to strain with your mind, your posture changes. For me, when I strain, my shoulders begin to hunch, the shakuhachi sinks down, and my chest area begins to close, just as if my body is literally closing up to protect my tired heart. If you notice yourself doing this, rather than fighting it, it's probably best to just take a break. Part of practicing shakuhachi is in practicing non-striving, which requires that you learn to notice when your mind and body are tired.
One way to supplement this practice is standing meditation, or Zhan Zhuang, which in Chinese means "Standing like a tree." This teaches you a way of standing (or sitting, of course) so as to allow your body and muscles to remain at once both loose, relaxed, and alert, letting tension drain away into the ground and allowing your life-blood to flow freely. It involves several different poses, two of which I'm going to outline here, as they're particularly applicable to learning to hold the shakuhachi without tension.
Firs of all, the usual disclaimers apply. This is a very gentle exercise, but the only one who knows your body is you. Be wise, and don't hurt yourself. If you do, I will not take responsibility for it, but I will offer empathy.
It's best if you can stand outside, preferably in the woods, a park, or another natural setting. If you can't, however, any area with enough space to stand comfortably will do. The meditation style is up to you - you can listen to music, listen to silence, let your mind wander, count your breath, recite scripture, or whatever you find most enjoyable and effective. Don't concentrate too hard. Smile a little.
Hold your hands out in front of you, arms rounded, as if hugging a tree. Your feet are spread apart, stable, about even with your shoulders, knees slightly bent but not to the point of going out past your toes. Keep your hands about chest level, about a hand's breadth apart, facing you, relaxed. Relax your elbows, but don't let your arms sink down too low. You might want to tuck your chin in a little, and look slightly downward to prevent yourself from straining your neck. Let it feel like a string is pulling the crown of your head up toward the sky.
Now, instead of hugging a tree, you are the tree. Let yourself be a little bit childlike and imaginitive. You have a wonderful imagination, and it's good to use it. Picture your roots in the ground, drawing up water through your feet, legs, trunk, up to your head and out to the "branches" of you arms, right out to your fingertips. Let it then cycle back down into the ground.
Now you can start to scan your body for tension. It's good to do this repeatedly as your practice - for most people, twenty minutes is a good length of time to practice, but even five minutes a day will make a difference. Tell the top of your head to relax, and then your eyes, nose, and mouth. Breathe deeply and naturally. Now tell your neck to relax, and then your shoulders. Let the tension sink down your trunk and out through your roots. Check to be sure that your hands aren't sinking too far down. Release any tension from your shoulders again, and from your elbows as well. Let your back relax - not too arched, but not completely flat either. Let it feel like your hips are directly connected to your legs. Your pelvis should feel open, letting blood flow freely. Relax your buttocks, and let the energy sink down through your legs to your feet and into the ground. Now go ahead and repeat the process again - after a while, this will become automatic, and you'll naturally learn to notice and release any tension that you're holding in your body.
As you're repeating this process, you'll learn more and more to relax into standing like a tree. An ancient Hebrew poet wrote that good people are like "trees planted by the water." What a good metaphor! The cosmos is full of love and life, the very stuff that you are breathing and that is running through your veins. Learning to live out of a place of rest is part of what makes us more "good," more human. As you stand, feel that energy cycling through your body with your breath. Let yourself feel connected to things. Let it feel as if your whole body is breathing. As you feel your body breathing, your arms and hands may begin to move apart and then back together in time with your breath, as if your tree is swelling and contracting with each breath. This is a good thing - it means that things are starting to flow well. You may also feel tingling in your hands and arms as well as your blood flow adjusts. Don't forget to let your face relax into a little smile. Your face can get kind of serious when you're concentrating, but smiling gently reduces facial tension.
Here, everything is the same as in the first pose, except for your arms. Instead of holding your hands out, you let them sink down to about belly level. When my wife was pregnant, before she gave birth, the hospital made the husbands who wanted to be present for their baby's birth take a class. In the class, they had us wear a "pregnant belly," a heavy strap-on belly that lets you feel what it might be like to carry around the weight of another small life. Imagine holding a pregnant belly, and you have this pose down.
This is also a good pose to relax into if your arms are getting tired from the first pose. If you want something to imagine, the tree image is probably the best. You could also picture your own pregnancy, though - not pregnancy with a baby human, but with something creative. The same creative energy that makes human beings is also present in you to create. What's sitting in you in seed form? What's brewing inside you, keeping safe until it's strong enough to come out and meet the world? What do you like? Some of us, myself included, have learned to stifle and reject their own creativity, sometimes even in the name of "morality" or "religion." "If I like it, it must be bad. I need to sacrifice myself." If you are going to sacrifice yourself, let it be your old, false self that doesn't know how to love. Make no mistake, though: you have creative gifts that it would be a sin not to express. You should never willingly sacrifice that, unless it is truly necessary for love. Let this be your time. Listen to the gifts you've been given, and let yourself dream.
Some may feel that this sounds too "new-agey," but there's no need to look at it that way. You're basically teaching your body to be active yet relaxed, so that your blood flows freely and doesn't get stopped up, making your back and arms stiff.
This is a great practice to supplement shakuhachi practice. Through it you will learn to recognize tension in your body, and to relax, or even withdraw from the situation or take a break from your work to let your body and mind come back to a place of rest. Another prophet said, "In return and rest is your salvation." If you want to know more about Zhan Zhuang, I would recommend the book The Way of Energy by Lam Kam-Chuen. Happy standing!