Talk, don't yell!

I've met many people who are great at parenting. One such person was the friend who talked to me about consistency. When helping me parent my child he would sit the child down and talk to him at eye level, making sure the child looked at him while conversing. During the conversation he would talk about what had happened and what changes could be made to give us all a better outcome.

The talks wouldn't go for too long because if the conversation drags on the child becomes bored and the point will be lost.

This is how I've dealt with children since.
Two children were playing computer games and the younger child was being encouraging and in awe of the older child's abilities. While the older child kept insulting and putting down the younger one.

I called the older child over so we could have a talk and before they left the spot they angrily demanded the younger one not do anything to the computer and began pushing and shoving, which resulted in a near fight and me having to intervene. By the time they came to me we had another issue to discuss on account of them not stepping away from the computer and coming over peacefully.

They stood next to me and I talked about their conduct and how things would have been better if they'd just put down the computer and came to me instead of getting in a disagreement with their sibling. Then I added, "But that is not what I called you over to talk about." They began defending their actions and so we discussed that in more detail and in the end we worked out that no matter what excuse was made, their actions of mistreating their sibling on the way over to me was unacceptable and unnecessary.

Again I said, "But that is not what I wanted to talk to you about." They stopped and listened to me as I asked them, "Have you noticed that your sibling keeps complimenting you, saying how good you are at the game, how awesome your skills are etc. while you continually put them down, insult and berate them?"

I didn't yell, I didn't raise my voice or my hand. Instead I explained the situation, pointing out an obvious thing that was going on between these two children, to which neither had been paying much attention to. This gave the older sibling something to think about. When they returned to play computer with the sibling they were more encouraging.

Even when I raise my voice with my child I rarely yell. Sometimes I will yell depending on the situation, although it is rare that I do so. Losing your cool as a parent means you've lost control of the situation and that is never a good position to be in.

Stay calm. Speak to your children like you would another adult. 

P.S. if you yell at other adults there's probably something wrong with you and you need professional help like anger management.