Speak loudly and firmly, with conviction. Try to make every sentence you construct a sentence you can be confident about. (Don't, for example, say that something is sort of true, or use a weak, hesitant voice. A rough physical argument is not just a sloppy version of some precise argument; it is a carefully formulated, defensible statement, beginning, for example, with ``In physical terms ...'') Use full sentences as you compose your thoughts or think of how to best express your next idea. If you realize you've made a serious error, or have gotten confused, say so, and try to straighten things out, out loud, with your audience. If you realize that you've said something in an unclear way, it's usually best to announce this fact, and that you'll now try to say it again. You can usually tell from your audience's facial expressions when they don't understand, and when you're going too slowly.
Be explicit whenever you can find a way to do so. Avoid ``this'' and ``that'' as nouns: say ``that tensor'' or ``that charged particle''. Don't say ``it'' for something that has a name. One can sometimes introduce an artificial explicitness . Thus, in a discussion, ``the ten-gram mass'' and ``the five-gram mass'' are better labels than ``this mass'' and ``that mass'', or ``'' and ``''.
Try to keep the audience informed of what you're doing, how things fit together, where you're going, etc. Thus, if you give an example of an argument, say before you start that it is an example, and after you're done that it was an example and that you're now returning to the main discussion.
Some of your comments will be vastly more important than others. It is vital that you indicate this relative importance. The following are techniques for emphasizing a point:
i) Say the point in a single, loud, short sentence,By using combinations of such techniques, try to get the correct distribution of emphasis in your presentation.
ii) repeat the point several times,
iii) pause after making the point,
iv) say ``The following point is important:...''
Do not allow your audience to get bored. If they look bored, try to drum up some enthusiasm. You might, for example, stop what you're doing and repeat, loudly, what your plan is, where you are in that plan, and why this problem is of interest. It is almost always a disaster to run over one's time. (The audience becomes bored and anxious to leave. Not only do they not learn anything after your time is up, but they tend to lose the thread of what went before.) If you see that your time is up before you 've finished, I would suggest that you stop there, and summarize in a few sentences. One often prepares a few additional points (e.g. examples) which could be worked into the talk, but which are not essential. One can decide during the talk whether or not to include these points, in order to make the time come out right.
Everyone has his own system for notes for a talk. I prefer a single sheet of paper with an outline of the talk on one side: the introduction, titles of the messages, and conclusion are the main divisions; the five or so points to be made in each of these divisions (summarized to a phrase) are the subdivisions. Occasionally, one jots an equation or two on the back.
In my opinion, the most important point about answering questions after (during) a talk is to be completely honest. If someone says that an argument does not seem convincing, and if you have doubts about it, say that it doesn't seem convincing to you either. If someone catches you on an ``omission'', say that you omitted it to simplify the discussion, and fill in the missing material. If someone asks something that hasn't occurred to you, say that it hasn't occurred to you, and whatever else you can. If someone asks a question in the middle of the talk, it's usually best, after the answer, to resume by working your way slowly up to where you were, saying the most distant material in a general way, getting more specific until you reach the point at which you were interrupted. If a discussion within the audience threatens to take over (in the middle of the talk), one can say (firmly) that he would prefer to postpone discussion of the issue until the end.
Talking about physics does not closely resemble thinking about physics because the purposes in the two cases are entirely different. The amount of information you emit is irrelevant; it's the amount you cause to be absorbed that counts. A talk has a clear objective, to force certain information into the minds of the audience. The idea is to direct one's entire effort to accomplish this objective. Surprisingly, experience in giving talks does not, after the first few, seem to make much difference in one's ability to give a good talk. What does make a difference, in my opinion, is having given serious and hard thought, over a period of time, to the art of speaking. Two times are particularly valuable for doing this: after you have just given a talk, and while you are listening to the talks of others. (Was the material properly arranged? What points were not clear? Why? What went over well, and what badly? Did the audience understand the plan of the talk? Were they bored? At what points, and why?)