Maybe it’s because you say something that gets a negative reaction. Maybe it’s because you begin to say things you don’t really believe or start making an argument that is beyond your depth. Usually, it happens when you are talking with people you respect and/or fear.

Here’s what you do:
The moment you feel you are losing it, end your conversation amiably and quickly with an honest admission. Say something like “Suddenly, I feel like I’m saying more than I should” or “I’m afraid I’m putting myself out on a limb here.” That should at least prevent things from getting worse.
Then, divert the audience’s attention elsewhere. If you are talking to people you like and respect, turn to one of them and say something like “John, you know much more about this than I do — maybe you can help me out.” If you are talking to a hostile audience, you could try something less risky: “Mr. Greenberg, I’d like to hear your opinion on this.” This tactic won’t absolve you completely all the time, but it will minimize the damage and quite possibly put someone else in the hot seat who will eclipse you (if you are lucky) in terms of making a fool of himself.
To prevent your getting into such problems, do this:
* Speak authoritatively only when you are a true authority. If you think you are an authority on many things, you are deluding yourself. Remember, too, that authority itself is relative. You may be an expert in your line of work — but compared to whom? When surrounded by colleagues who have the same amount of experience you have, be more careful about your statements.
* Speak tentatively about subjects you know about to some extent but don’t know cold. Express yourself with questions instead of statements. Use qualifying adjectives and adverbs.
* Keep in mind that you will impress people most by talking less yourself and getting them to talk more. These are probably the most important rules about self-confidence.
But there are also some relatively small things you can do — some little tricks that can help you appear to be self-confident:
* Speak in short, distinct sentences.
* Look directly at the person to whom you are speaking.
* Periodically, ask questions — specifically, questions that will provoke positive answers.
* Smile.