Do not attempt this unless you are in very good health see the disclaimer
Now take a nice, natural, big, deep breath, and exhale again. Do this a few times. Note the word "natural". Again, we brass players can tend to loose sight of the most natural things. Breathe as though you were lightly winded after a short, easy sprint, or as if you just opened the window and are tanking up on good, fresh air.
Don't let your chest collapse downward as you exhale. Stay tall as you blow. This point is important, and will make the next inhalation easier, more natural, as the next breath doesn't have to help re-inflate the collapsed position.
And, a final posture point: I believe that posture often shows how you feel about the current task, or even about yourself. Are you crouched and squinting into the music stand? Stand tall and confidently - even if you have to fake it at first! I think you will find that feelings follow actions in this sense. Or, to put it another way, "Confident is as confident does." Sometimes we have to act the part before we feel we own it.
Yoga views a person’s posture as a physical manifestation of one's inner state. One's view of the world and one's mental, emotional and spiritual state reflects in one's general deportment, including how one postures oneself. Deficiency in posture often begins in childhood with lack of awareness which becomes habitual and self-sustaining. This pattern becomes further reinforced and perpetuated by the stress in our lives and chronic neuromuscular tension. Yoga can change this.
Conscious static stretching, (the various original postures of classical yoga, about 20 in number) is the first step. This is how you begin to penetrate and disintegrate the old status quo, that is, the chronic patterns of neuromuscular tension which have been sustaining less than optimal posture. Important to note: yoga poses should always be adapted to the specific individual doing them; in classical yoga, stretching is always done gently, in a natural manner biomechanically, and with a feeling of relaxation. If it hurts or makes you sore afterwards, or if it feels wrong, it means something is wrong. "Not strong, not complicated" is the adage in classical yoga.
To increase your benefit it's important to understand that the stretch is just the beginning of a process which needs to be completed. The stretch creates a magic moment: by releasing tension, the stretch makes the neuromuscular system receptive to positive change. You want to take full advantage of this. Right after your major stretches i.e. several times each class, you simply recline on your back and, with eyes closed, you completely "let go". You drop off into a sort of "half-sleep, half-awake" state where the relaxation feels so good you just don't want to move. It's known as "deep alpha" in biofeedback, referring to slowed brain-wave activity occurring during deep meditative states. This is the secret. Yoga stretching combined with such deep relaxation enables a natural re-organization of the tension holding patterns within the neuromuscular system, one benefit being better posture. This is the key which awakens yoga's legendary benefit to the fullest.
Purists within the yoga community will go so far as to say that yoga poses done without such deep relaxation is not yoga! These magic moments occur several times each class. You emerge from such an experience feeling lighter, better, focused, energized. And this benefit can trickle into every area of your life, all day long, reflecting in part as how you posture yourself. This is why it is said that yoga affects your whole life.
Complete yoga breathing, done during the class as well as habitually during the day is a way of carrying yoga with you. It stabilizes your focus, part of which is maintaining awareness of how you posture yourself. Hence the saying: “Good breathing means good posture.” Breathing well throughout the day, plus sitting tall, standing tall and walking tall all become positive habits in your life.