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Exercise regularly, just before a stressful situation, or as a response to anger. Research has shown that exercise tells your body to release helpful endorphins that reduce your body’s stress response and elevate your mood. All of these can help you calm down. Exercise of any kind can have the same helpful effects on anger, so do an activity you typically enjoy (such as running, dancing, or walking). For example, if you know you have a stressful business meeting later in the day, take some time in the morning to go for a run. This will reduce stress and tension, so you are not walking into your meeting wound up. Although it sounds odd, counting can actually help to slow you down when you are tempted to explode or have a tantrum. Focus your thoughts away from the situation and on to your counting and breathing. Count to 10, taking a breath with each count and in between each number. Doing this at a slow pace can help you slow down and become calmer. Continue to count if you still feel angry after reaching 10.  When you're angry, your body releases adrenaline which increases your heart rate, makes you feel flushed, and causes other physical signs of anger or arousal.  Counting gives you a chance to slow down and calm your reactions so that you don't act impulsively. Learn to breathe with your diaphragm, the muscle at the lower part of your lungs. Focus on your breathing and take full, deep breaths that fill the entire lung. Breathe in for 5 seconds, then hold for 5 seconds, and release for 5 seconds. Take 2 normal breaths in between each diaphragm breath, so that you do not hyperventilate and repeat the exercise until you feel calmer.  Breathing signals your body to calm down. It does this by releasing neurotransmitters that tell your brain that you’re ok and safe.  When you're angry, adrenaline can increase your breathing rate, making it feel as though breathing is difficult or that you can't get a full breath. Meditation can help you concentrate, reduce your stress levels, and enable self-awareness. All of these things can calm you down when facing an emotional outburst. Try to take at least 10 to 20 minutes a day to practice mindfulness, which can improve your overall mental health. There are several forms of meditation you can try, like Tai Chi, yoga, transcendental meditation, and Qi Gong.
Exercise. Count. Do breathing exercises. Meditate.