How we change what others think, feel, believe and do
Here is some advice on what not to do when you are speaking in public or making presentations.
Don't read things out to the audience. This means you should not read a written speech, although some who speak a lot (and long) such as politicians resort to autocues and other 'cheats'.
When you display slides you should not just read out the words--the audience can read this themselves. In fact when you put up slides with words on it is often best to pause to allow the audience to read the words (which they will do anyway, ignoring what you are saying).
Reading things out is disrespectful of your audience. It says 'You are so unimportant to me I did not learn the words'.
If you act as if the audience is not there, for example by talking to the back wall or generally with de-focused eyes, you are telling them that they are not worthy of your attention. Some speakers do this to avoid the discomfort they feel when being watched (which is often based on a fear of being judged). A few actually do feel too superior to acknowledge them and look around as if there are more interesting other things to look at.
Turning your back on audience is also disrespectful. It generally says 'You are so unimportant I can ignore you'. This can be done by accident when you turn to point at the screen behind you (this also makes you much harder to hear).
A generally useful rule when speaking is to stand straight up and keep your hands out of your pockets. Hands are useful in presentations to communicate with larger gestures so those at the back get the message too.
There are some contexts where being casual is acceptable and maybe even desirable, such as at 'geek conferences', although even these have an intense desire for sharp content.
We have a tendency to fill the gaps between phrases and sentences when we are thinking of what to say next with words and sounds like 'um', 'ah' and 'er'. This makes you sound uncertain and also makes it more difficult for your audience to understand you.
A purpose of 'ums' in conversation is to send a signal to others who might interject that you are still talking. This is not needed in public speaking as nobody is going to interrupt you.
When you are nervous, this often comes out in sudden gestures and whole-body twitches. You should not look like a marionette being controlled by a poor puppeteer as this conveys your nervousness and either makes the audience empathetically nervous too or exasperated at your lack of control.
Also avoid 'verbal twitching', such as making clicks, little coughs, lip-smacking, clearing the throat, nervous laughs, biting the lip and so on.
The best way to avoid twitching is be relaxed, and there are ways you can help this before you start. If you are still jumpy, do try to control yourself. Force yourself to stand in one position by 'planting' yourself, rather than dancing around with 'happy feet'. Hold your arms steady or slow them down rather than doing a 'windmill'.
When you are nervous, a reaction can be to speed up, subconsciously trying to get to the end sooner. When you talk quickly, though, it is more likely that people in your audience will miss what you say and so become lost (and hence annoyed).
If you are conveying complex ideas, then people need time to digest them. You understand them already so may not feel the need to slow down, but the general rule is to 'talk at the speed of understanding'.
If you have slides containing text, people will try to read them. If you remove the slides before the people have finished, this will likely irritate them. One secret is to use less words. Another is to give plenty of time for the slower people to read carefully.
The opposite of hurrying is dawdling, talking slowly and casually as if you have all the time in the world. This is likely to bore people who want value from you and do not like wasting their time.
A classic method of dawdling is the digression or side comment. If you go off-topic, do so only briefly and do not ramble away about things that interest you but just leave your audience yawning.
Generally avoid apology and particular when it just makes you look incompetent and uncaring. For example you should never start with something like 'Sorry but I didn't have time to fully prepare' as this tells your audience that they are unimportant to you.
If you say something that is incorrect and especially if this is pointed out by someone, then it certainly is appropriate to apologize, but in such cases do so clearly and briefly. Blustering on when you are clearly wrong only makes you look foolish.
Always, always be respectful of your audience, even if they are rude, stupid or uncaring. First, be well-prepared. Then speak to them, not at them. Talk with authority, but not arrogance. And listen carefully to any comments you receive.
If people criticize you, even personally, do not descend into a slanging-match as you are the person who will look bad, even if you did not start it. Take the high ground. Respect their views and their right to hold them, even if you disagree profoundly with them. Ignore personal criticism. Don't lie. If you don't know, say so (and maybe promise to find out).