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One of the first and easiest steps to warm up your body and your throat before you sing is to open your throat and diaphragm by yawning.  Gently force yourself to yawn by opening your mouth as though you were about to yawn. As you do this, think about yawning, or watch a video of someone yawning to initiate your own yawn.  Repeat this two or three times to fully open your throat and diaphragm. You can also open up your throat with some light physical activity. Try doing some quick exercises, like jumping jacks or a short walk or jog, take a short break, and then do your vocal warm-up. Using your abdominal muscles and singing from the right place in your body are very important when you sing. To engage the muscles you should be using, very gently force yourself to let out a small cough. Pay attention to which muscles are involved in that action, as these are the muscles you should be using when you sing. The muscles of the core include the psoas, the pelvic floor, and the diaphragm, among others. Engaging these muscles during singing will help you achieve your fullest voice. You want your entire body to be relaxed when you sing, because you don’t want to strain your body or your muscles when you sing high notes. To relax your upper body, simply shrug your shoulders, hold them in a hunched position for five seconds, then relax them. Repeat four or five times.  Your voice should always come from your diaphragm, but sometimes people will try to push from a higher place on their body instead of the belly when reaching for high notes. To prevent this, continue relaxing your neck and shoulders throughout your warm up, especially when going for high notes. Because the breath is the mechanism that creates your voice, it’s also important to do a few breathing exercises before singing as well. Two exercises you can do are:  As you keep your shoulders and chest relaxed, inhale deeply into your diaphragm, so that your stomach rises slightly. Then exhale slowly from this same place, so that your belly flattens out again. Continue breathing this way for two minutes. Inhale like before, but when you exhale, practice allowing the breath to escape slowly as you make a hissing sound with your mouth. Repeat for one minute. Tension in your jaw and mouth can also affect your singing, so relax this area before singing. To release this tension:  Place your palms on your cheeks and allow your jaw to open to a natural position. Slowly and gently move your hands around to massage the jaw and facial muscles for one to two minutes.

Summary:
Open up your throat. Engage your core. Relax your neck and shoulders. Do a breathing exercise. Release tension in your jaw.