At the beginning of each message, state again what the title of the message is, that you're now beginning on that message, and, if you like, a few sentences about the content of the message. At the end of each message, state that you've finished the message, and give your few-sentence summary of it. (That is, sentences which begin ``We now begin our discussion of...'' and ``To summarize,...''.). These summaries, particularly, are very important: they give your audience a chance to think over what you've said, and to draw it all together. (They also provide an opportunity for those who have gotten lost in that message to again pick up the thread of the talk.). This transition from one message to the next must be abundantly clear to the audience. One can also repeat, between certain messages, what the plan is, where you are currently in that plan, and how all the messages will tie together. It is crucial that, at every moment during the talk, your audience knows what the plan is and where they are in that plan.
The body of each message should be a short talk in itself, with a clear, central objective. (Thus, for example, if you use in one message material from an earlier one, you should briefly summarize that material first.). A message normally consists of three to six points you want to make.
The mode of presentation of a message is not normally the way one thinks about it privately. In particular, one should try diligently to suppress everything which does not bear directly on the central objective of the message. Examples:
In short, avoid everything which leads away from your message. It is even sometimes necessary to say things which are (technically) incorrect, or which contain omissions, in order to accomplish this objective.