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Hold yourself upright. Keep your head up. Shift your weight slightly forward. Open your mouth wider when you sing.
Push your chest out and keep your shoulders back. Relax, but do not slouch forward. Be confident. Consciously hold your shoulders back, and catch yourself when they hunch up around your ears. Let your shoulders pull your chest up, slightly, making room for your lungs to hold more air.  Don’t force it. Control your posture, but not so much that you are rigid. Keep your shoulders as far back as possible, but don't make yourself uncomfortable. Flatten your belly, but do not flex your abdominal muscles. Be upright, but relaxed. Do not bend forwards or backwards from the waist. Stand up straight so that your shoulders are above your ankles. Gaze straight forward so that your chin is parallel to the floor. Avoid looking up or down when you sing, lest you constrict your vocal chords and limit your range. Try to keep your throat as open and straight as possible – do not twist or stretch your neck out of its natural position. Stand with your feet about six inches apart, toes facing forward. Place one foot slightly further forward than the other. Rest on the heels and soles of your feet, and make sure that you are stable. Keep your knees and your elbows slightly bent. Let yourself relax into a comfortable and natural singing position. Many singers let their mouths close slightly as they sing, which can muffle the effect. Less space in your mouth means less sound coming out of your mouth. Open your mouth to lend resonance to your voice! Use the "two finger rule" as a litmus. When you are singing open vowels like "ah" and "oh," try to fit two fingers between your teeth. If you can fit two fingers between your teeth, then your mouth is open wide enough. If you can’t fit two fingers: open your mouth wider.