You've been asked to give a short talk. How do you prepare?

As you might have noticed, this is my fish market speech. For the first few minutes I flounder. - Bob Orben

Whether you're giving a committee report to your church or selling the executives in your company on a new product, chances are you've had to learn to brief a group on a subject where you're considered the expert.

Before your next talk, consider the following ardent rules taken from an 1899 book on oration:

* A man must always stand with his feet at an angle of 45 degrees, one foot slightly in advance of the other, with his weight resting on the balls of his feet.

* When there is a change of thought, or transition from one topic to another, the speaker should always take one step (but not more than one) backward.

* When a major point is to be emphasized strongly, the speaker should always take one step forward (again, no more than one), after which he returns to ''home position.''

Orator aerobics like these may not do much for your briefing techniques, but Katharine Bruce, a management consultant who teaches briefing and presentations in the Washington, D.C. area, has some better ideas for giving a talk. She thinks you should start by asking yourself why you're giving the briefing - what you hope to accomplish with it.

''Don't confuse the objectives of your briefing with the objectives of the content of your briefing,'' she says. ''For example, if you were trying to persuade your executives to start a new product, the objective for selling the product might be to get new customers or make more money. But the objective of your briefing is to get the executives' approval - that's your real objective.''

Once you know what your real objective is, she says, ''you'll know how to aim your speech.''

Caryl Winter, author of ''Present Yourself With Impact,'' thinks the way to get started is to ask yourself, ''What are the needs of my audience? What do they want to hear?''

The audience is a large part of a briefing, Ms. Bruce says, and you should get as much information on it as you can gather. What is your listeners' knowledge of the subject? Can they absorb a lot of technical information? Are they favorable or hostile toward your subject?

By talking with others who have spoken before this group - or by talking to whoever asked you to make the presentation - you might also find out if the audience likes its information in figures, facts, anecdotes, or visual aids (although Ms. Bruce thinks every talk benefits with the latter).

Once you know what you're aiming to accomplish and whom you are aiming at, you can start to organize your material into a brief speech. Most people already have their favorite technique of organizing material, writes David Lewis in the ''Secrets of Successful Writing, Speaking & Listening,'' ranging from speaking randomly into a tape recorder to making short notes on cards. You'll probably have to do some research on your topic, ''though you may already have selected some resources,'' Ms. Bruce says.

Although there's no ''time formula'' for organizing the briefing, she says, most briefings can be roughly divided into a short introduction, a meaty body, and a quick conclusion. ''Start off with an anecdote or vivid detail - something to get their attention,'' she advises, ''and then go right into your body.''

For Mr. Lewis, it's important to get your ''credentials'' in early, proving to the audience that you have ''earned'' the right to give that presentation. As a general rule, he writes, you should know ''more about the subject than at least 75 percent of your audience.''

Keeping it short means keeping it to ''no more than five main points,'' says Ms. Bruce, who adds that ''there are 101 different ways to organize them. You can go from cause to effect, talk about them in a problem-solving way, go from small points to big ones, or the other way around.'' She also thinks you should consider what you want to leave out: ''If you're going to have question and answer sessions, you might want to keep some of this information in reserve,'' she says.

The conclusion is typically ''the weakest area of any presentation,'' she notes, though it should be the strongest - ''this is what you want them to take away from your talk.'' She suggests a story-board technique, using flip charts to outline your talk as you go. As you get to the end of each page, tear it off and hang it on the wall in front of your audience. Then, at the end of the talk, you can literally walk through the speech again, pointing out the major areas covered.

The experts all advise practicing your speech out loud in front of a mirror or, preferably, in front of friends or relatives. ''Most people think their friends are a tougher audience than a roomful of strangers, for some reason,'' says Ms. Bruce, ''and if they feel comfortable giving the speech to their friends, chances are they'll feel comfortable when it comes time to talk to the bigger group.''

When that day comes, Ms. Winter says, you should ''choose clothing that is comfortable and flattering, and that won't get in the way of the presentation,'' take a deep breath, and ''relax and enjoy it.''

''Look at it this way,'' she writes. ''If you think your idea is good, then you should want to tell others; or if the request to make a presentation comes from others, would they ask you to do it if they didn't think you could?