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Most people communicate just fine, yet most are extremely uncomfortable speaking in front of crowds. This is usually caused by two problems: presentation, and confidence. There's a third cause, topic, but usually you don't get to choose that. Howeve,r you can often make up for a boring topic by having a good presentation, and a lot of confidence.
Presentation includes a lot of things, from having the right "hook", to a bit of theatrics, humor, involving the audience, and so on. Most public speaking courses focus on presentation techniques, and you can pick up plenty of books on that topic, such as "scan the room in concentric patterns, slowly, from outside in", "present the proper body language", and so on. While they are important, they are practical skills. In terms of the military, presentation techniques are tactics: what you do DURING the battle. You also need to have the proper strategy: what you do BEFORE the battle. That's where confidence comes in.
Confidence is more than simple courage of standing up on stage and face the crowd. A lot of people think that confidence is same as self-esteem. That is VERY WRONG. No amount of mental tricks will give you self-esteem when you have none. Confidence is knowing the material, having practiced the delivery in rehearsals, unlearn any bad habits you may have picked up before, and basicaly speak from your heart, about something you believe in. The problem is most people somehow attribute these things to born talents, like "born speaker", when in fact they have just picked up some very bad habits along the way.
A lot of problem in communication starts young, VERY young. Younger than most people expect. In fact, many problems in communication are best solved by therapists, not speech coaches. Thus, communication problems are NOT solved overnight. There is no "aha" moment where you flip a mental switch labelled "great orator" on. That is simply not possible. In fact, there are a LOT of self-denial going on when it comes to public speaking. Often, you'll hear stuff like "It's not me / not my style / not in my nature" or similar excuse. Frankly, that is... BS.
There is no such thing as "my nature". I am NOT going to debate "nature vs. nurture". It's clear that you do have genetics, and they do influence you. However, what genetics do is give you a set of tinted lenses for you to see life's experiences. It's the PERCEPTION (alread tinted) of these events that shapes your mind. And with enough training, you can see PAST the tint to the real events. On the other hand, it is also possible to look at the tint, and see things that are NOT THERE. And that is where problems develope.
All of us are shaped by experiences we picked up when we were young, all the way until we die. It is simply the way we live. And each of those experiences are "tinted" by our genetics, and previous experiences.
If our taste buds are not that sensitive (regular, instead of "super-tasters") then food doesn't quite taste good to us than to the super-tasters. Same food, different experience. If the food was served in a romantic restaurant with an enchanting partner, vs. someone you can barely tolerate, same food, different experience. If you have had good experience in this retaurant vs. a bad experience, and you came back, same food, different experience. Your perception of the current event is tinted by your prior experience, and genetics.
Thus, while genetics may influence your confidence, don't BLAME it for your lack of confidence. It is merely one more factor you have to compensate for, like audience, topic, facilities, and so on.
Another component of confidence is familiarity of topic material. If you know the topic really well, you will be far more confident than if you are not familiar with the material. However, there's no such thing as "material speaks for itself". Even a brilliant topic can be sunk by a sub-par presentation. And because you are the one speaking, people see the topic via your performance. A great speaker can make mundane exciting, and a bad speaker can make exciting mundane.
Yet another component of confidence is self-esteem. Self-esteem is one's faith in one-self. Self-esteem is NOT confidence. While presentation skills can easily be taught, lack of self-esteem is an endemic problem that takes time to resolve. People with low self-esteem often do NOT take criticism well, and can be quite belligerent, as they have learned the "best defense is a good offense" technique as a coping mechanism. They need to unlearn the coping mechanism to communicate effectively. In fact, very often, people who appear to have the most self-esteem ("I don't have a problem!" "I am doing just fine!" "I don't need any help!") actually have the least, but that's a problem between them and their therapist.
Yet another component of confidence is proper body language. We all can spot people who looks insincere. Their body language and their eyes give them away. They don't fully believe in what they are saying, and it shows in the body language. You would not trust a TV news anchor that reports even grisly murders with a grin on his or her face, would you? On the other hand, one can also get into the habit of using bad body language all the time. When the body language do not match the verbal language or facial expressions, the audience knows you are not being sincere.
Public speaking is a large subject, and many speech coaches don't learn about the players and determine the true cause of the communication problems, but instead present sound-bytes that are short, succint, but may or may not apply to you. To truly upgrade yourself, you need to know yourself.