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Relax your jaw to make your speech a lot clearer with some exercises.  Make wide chewing motions while humming gently. Stretch every muscle in your jaw and face. Open your mouth as wide as possible (as if you were about to yawn), while moving your jaw in circles, and sideways. Open your mouth wide, as in the previous exercise, and shut it again. Repeat 5 times. Make a buzzing sound with your lips together, but don't clench your jaw. Just like breathing, your posture plays a large role in your speech clarity, and it's something that we sometimes forget to take into account.  For best speech clarity you should stand straight with your shoulders back and your weight evenly distributed. Do some shoulder rolls and standing side bends to help with your posture.These simple exercises will also help you with your breathing, and you can even do your jaw strengthening techniques while you stretch. Doing vocal warmups will loosen you up and prepare your voice so you can speak clearly and effectively.  Even if you're not singing, you can do some scales or just hum. Try singing your tongue twisters too. Do some “Whooos!” going up and down. Picture your voice like a ferris wheel going round and round. Hum and pat your chest. This can help knock out any phlegm you may have. When you clench your teeth you're causing a lot of strain on yourself and it can be a sign of stress. And speaking with a clenched jaw will prevent you from opening your mouth enough to enunciate and speak clearly. If you find yourself clenching your jaw, practice some of the warmups, and take a moment to take a deep breath slowly exhaling by letting your breath buff your cheeks out, as if you're letting air out of a balloon. Your vocal chords are like a machine that needs to stay well oiled. Gargle some lukewarm water with a pinch of salt to keep them limber. This helps relieve tension in the throat.
Practice jaw exercises to enhance clarity of speech. Watch your posture. Warm your vocal chords. Avoid speaking with clenched teeth. Stay hydrated.