Dorothy Sarnoff, a leading New York speech consultant, has given Prime Minister Menachem Begin some fine points on how to respond to television interviewers. She has rehearsed former President Jimmy Carter on a State of the Union message. She is also the communications consultant to the State Department.
In addition, she has helped more than 50,000 other people, from 17-year-old high school boys to corporation presidents, present themselves and their ideas with more confidence and authority than they ever thought they could muster. Sometimes even whole families have flown into New York to sit down with her and learn how to speak and listen to one another better.
Miss Sarnoff, whom many will remember as a costar with Gertrude Lawrence and Yul Brynner in the first Broadway version of the musical ''The King and I,'' deserted the stage in 1963 to devote herself to helping others say well what they want and need to say.
It was about then, too, that she decided to let the words of Harry Emerson Fosdick, the longtime minister of Riverside Church in New York, dictate an important part of her approach. Those words were, ''Have the daring to accept yourself as a bundle of possibilities and undertake the game of making the most of your best.'' This sentiment also determined the title of her new book, ''Make the Most of Your Best,'' (New York: Doubleday & Co., $14.95) in which she shares many of the techniques she has refined in private consultations and seminars held all over the world. The author is the chairman and founder of Speech Dynamics Inc.
It was the book that occasioned my interview with Miss Sarnoff in her teaching studio high up in Steinway Hall on West 57th Street. The blond former actress-singer wore red, a color she likes a lot and one that seems to match her own vibrancy. One can see at the outset that she clearly retains what even her competitors term ''star'' quality. It works in her favor. The aura comes through as a comfortable empathy and helps give authority to her work as a speech consultant.
To interview Miss Sarnoff is to get a rather breathtaking performance. She doesn't just answer questions, she acts them out. She demonstrates her points, jumping up to the podium with a ''let me show you the difference.'' Then, with her voice, her posture, her eyes, she illustrates her points.
But what about people who will never have to speak at any affair more prestigious than the local PTA or women's study group or Lions Club? The principles that apply are always the same, she replies. They include the following:
* Intelligent anticipation. Don't wait until the last minute to prepare your talk carefully and intelligently. Try to visualize what the situation demands and take all the time you need to get ready for the big moment when you will be up for inspection before an audience. Jot down the main theme of your message, such as ''saving energy,'' then build on it. This will help you edit your thoughts and focus your speech.
* Always know who your audience is, and direct your talk to that particular group. Every audience enjoys feeling that your talk was fashioned especially for it.
* If you are rather inexperienced, you cannot lose by writing down every word you expect to say. Then check to see if you have the right structure, all the facts, and enough color. A tape recorder can help at this point.
* As you are putting your speech together, be sure to open with something snappy that will make an audience sit up and take notice. Never, ever use ''ladies and gentlemen.'' Remember that analogy teaches faster than anything else, and that anecdote colors and gives you the authority of evidence, preferably personal evidence.
* Set a familiarization period, and speak your words to determine if you have written yourself a ''spoken speech'' or an essay. Then rehearse at least four times aloud as though you were in the real-life situation, the final time as near to delivery as possible. By that time, you will know your material so well that occasional glances at the page will enable you to see key phrases that will keep you moving through your speech.
* People the room with an imaginary audience and sweep them with your eyes. Do not read, in a head-lowered position. ''Eye talk'' to the audience, even to those in the farthest corners. They will feel your intent to make visual contact with them, even if the hall is so big that you cannot do so, and thus perceive your sincerity and credibility.
* Remember that if you do not speak with joy, energy, and enthusiasm, you will bore people. If you read to them, you will insult them. If you grope and fumble, you will not only bore them but make them feel uneasy. If the words you speak don't have ''the ring of truth'' and of personal conviction, you will not only bore, but come over as a phony. If you don't keep the generator going vigorously behind your speech, you let down its momentum and lose your audience.
* Love your audience. Many people think of themselves as being shy or nervous when asked to speak, but four little sentences are foolproof for keeping speakers in a positive, poised state of mind. They are: ''I'm glad I am here. I'm glad you are here. I care about you. I know that I know.''
Miss Sarnoff says the most common fault of speechmakers is the failure to edit and tighten their speeches sufficiently. Speakers, she claims, often don't get to the point soon enough, and they clutter their talks with unneccessary details.
''I don't teach gestures,'' she comments. ''My feeling is that if you gesture naturally, then do so.''
A video camera is a chief teaching tool in her studio, enabling clients to size up their own performances. ''That picture is worth millions of words,'' she says. After they see themselves in action they want to change their speech, and often their appearance and behavior as well. They love comparing their ''before'' and ''after'' film sequences and enjoying their improvement.
If $14.95 seems expensive for a book, bear in mind that Miss Sarnoff charges