President Reagan isn't the only executive who likes issues spelled out in one-page memos - or the first to learn the creed set forth by David Belasco, the American theatrical producer: ''If you can't write your idea on the back of my calling card, you don't have a clear idea.''
Properly used, the memo is the great interoffice tool for brainstorming and barnstorming. It gives the audience you're trying to reach something concrete to grapple with - and feeds it to them so ''it doesn't come as a surprise,'' says Priscilla Taylor, editor of a communications newsletter called the Editorial Eye. ''If you just call up someone with a new idea, you may not get a chance to tell him all the details. And he may absorb them better in writing anyway.''
With memos costing at least $10 each in employee writing and typing time - and sometimes far more - it's important to use the tool carefully. To be effective, the memo needs to be clear, with your objective simply and quickly stated.
Marya Holcome and Judith Stein, authors of a new book on corporate communication, suggest that you ask yourself three questions first: Is it clear why you're writing the memo? Are you sending it to the person who will take action on it? Do you make your point in the first two sentences?
For Deborah Dumaine, founder of a consulting firm called Better Communications, the question to ask yourself is more direct: Why are you writing this? What do you hope to achieve by it? If you can't state your objective clearly, she says, chances are you need to think it through more and perhaps do more research.
John Morris, inventor of the ''behind the words'' system for better communication, points out that knowing the objective is not enough - you must know your audience as well. Otherwise, he says, a note to terminate an employee would read, ''You're fired'' - a message that certainly accomplishes the objective, but fails utterly to consider the feelings of the person receiving it.
He also thinks memo writers should watch out for a ''hidden audience'' that might receive their messages indirectly. ''Sometimes they don't have the power to take affirmative action on your message, but they do have the power to block the action you desire,'' he warns. To avoid this, he advises bringing the audience out into the open by sending them copies of the memo, or at least bearing them in mind when you're writing it.
With a clear understanding of both your objective and your audience, Ms. Dumaine suggests you start writing down the thoughts and research you want to include in the body of the memo: pros and cons, supporting data, background information, explanations of a process, results of a study, other considerations.
Put these into some comprehensible format - listing your main points from most to least important, perhaps; listing general parameters and specific problems; or listing the information according to location (which office furniture goes where, which branch offices are producing what).
If you are comparing two or more solutions to a problem, Ms. Dumaine says, it's better to show the advantages of both solutions and then the disadvantages of both, so the reader doesn't have to jump up and down the page to weigh the two (or three or four).
Ms. Taylor says the best way to write this sort of memo is to present the solution you think best in personal terms: ''Say, 'I found it!' Using 'I' and 'you' lets you use the active voice - it's stronger and more direct. That's always the key in memo writing.''
Once you've written out your information and arguments and have an idea of format, ''plunge ahead,'' Ms. Taylor says, roughing out a first draft. ''Don't think this is a one-time effort.''
Vivian Vahlberg, past president of the National Press Club, has good advice for those with writer's block: ''Lower your standards and go on. It's always easy to go back and improve; what's hard is getting down the first draft,'' she says.
Once you've got something on paper, look it over in two ways: First, see how ''scannable'' it is. Can you pick out the main points easily? Consider underlining these for clarity, or indenting them with bullets to make them stand out. ''You need plenty of white space on the paper,'' Ms. Taylor says.
Next, ''fry'' the fat out of it, putting in active verbs and taking out ''pompous polysyllables,'' gobbledygook, in-house jargon, negative sentences, and any extra words you can find. ''When I see a paragraph shrinking under my eyes like a strip of bacon in a skillet,'' says writer Peter de Vries in a Life magazine article, ''I know I'm on the right track.''
Finally, check to see if everything is written in the most positive light, in a way that won't trample on either those reading the memo or those affected by it. Enthusiastic and informative, the memo should, as a former editor at National Geographic puts it, ''light a fire under the readers - not torch them.''
It's that kind of effectiveness that can be achieved with clear memo writing.