The mails received by conductors of today's symphony orchestras are regularly filled with packages of various sizes and shapes. Usually wrapped in brown or white paper, they arrive in streams that never stop. Inside the packages are the hopes and dreams of hundreds of composers, usually on the younger side, who are wondering whether the lightning of luck will strike this time.

Many conductors can point to a stack of these packages that may rise to four or five feet in height. Some may have been there as long as six months to a year. What is inside them?

You might think each one would contain a score, the full orchestral score of a huge success if a conductor will properly rehearse and perform it. But when is the conductor -- with rehearsals and concerts filling his days -- to find the time to look through these scores? Or to give each one the kind of time and careful attention that is needed if he is to get a clear idea of the value of the music?

To help with this endless but important job, most conductors have reading assistants who go through the piles of scores to pick out those that look promising. And you would be amazed at some of the things they find -- and do not find.

Andreas Makris, a violinist in the National Symphony Orchestra who is also a composer, performs this function for Mstislaw Rostropovich. Makris also is a composer whose "Chromatokinesis" will be played on the NSO subscription concerts on Jan. 8, 9, 10 and 11. He was talking the other day about the things some composers do or do not do that can make a big difference in what happens to those scores they send in. After a while, he drew up a list of guidelines that every composer should check before mailing out his latest hope.

Makris suggests:

1) Send only scores. Do not send parts. they have no vale and can easily get lost. Since concerts are planned from one to two years in advance, there is no rush about getting the parts.

2) Do not send samples of scores. Makris says he often receives only the first page or the first two or three pages of each movement of a four-movement symphony. This does no one any good whatsoever.

3) Do not send tapes only. A full score is the only thing that is looked at seriously. (Here Makris is taking it for granted that all conductors can read scores, an assumption to which the history of orchestral music in this country has had some rather startling exceptions!)

4) Do not send condensed scores or piano reductions. Again, the full score is what is essential.

5) A poor tape is a minus rather than a plus. "A poor tape can mean instant death."

6) Sending a work composed for your doctorate is not working to your best advantage, especially if it was composed 10 or 20 years ago.

There are, of course, exceptions to this rule. Samuel Barber wrote his immensely witty and justly successful overture for "A School for Scandal" as a graduation exercise for finishing his work at the Curtis Institute.

7) Sending fives scores is no help. One or two is perfect.

8) A printed score is no more impressive than a manuscript, Makris points out -- a suggestion that could save money. "We are looking for fine music," he adds, "not particularly financially successful music."

9) Scores written in the past 10 years or so are preferable to scores written at an earlier date. (This is arguable, in light of some music that has been neglected for years. If it's good, what difference does it make when it was written?)

10) Generally speaking, send only symphonic music to an orchestra. A song cycle, a cello sonata, pieces for piano alone, or a string quartet are not the right music for an orchestra.

11) Send the complete work, not a single movement or a part of the work, saying that the rest will be finished in a year. "We will simply wait for the work to be completed before looking at it."

12) Generally speaking, sending chamber music will diminish your chances of being played. Most modern conductors conduct very few concerts of this type.

13 if you are an unknown composer, keep in mind that it is harder to promote very long works, or works of unusual combinations such as triangle, bassoon, and carnatic violin. And, Makris says, "believe me, we get these!?

Having spent years worring about why some composers do the things they do, Makris has come up with these suggestions: telephone calls trying to explain or analyze your music are no help; a one-page biographical sketch is much better than a 35-page booklet.

And he has a final suggestion that you might not think necessary. You would be wrong. "Always put your return address on the score," Makris insists.

William Smith, who is Eugene Ormandy's assistant conductor with the Philadelphia Orchestra, and who reads through the mountains of scores that reach that office, told me, "We have a green cabinet full of scores -- some of them go back to the '20s. They have no return addresses."

"And not only with no return address," Smith added, "some of them come in 'postage due!'"

He talked about the small number that have any chance of succeeding. "It is sad, but they are relatively seldom usable.Some composers are simple illiterate musically, and yet it often seems that the less talent they have, the greater their egos.

"And when we do not play their music, they send insulting letters suggesting that we are passing up another Beethoven."

For some composers, however, the process of packaging and mailing scores to conductors who are known for their sympathies toward new music pays off. Only last season Antal Dorati programmed a new work by a young West Coast composer whose name was completely unknown.

"It just came to us in the mail," Dorati said, "and when I studied it I thought it was a good piece and that the audience would like it." He turned out to be right on both counts.

Orchestras' score readers across the country tell the same story Makris has told. The New York Philharmonic has a history of turning over the stacks of scores to its assistant conductors for preliminary reading. However, David Gilbert, the Philharmonic's last assistant, left the post several months ago. gSince that time, they have had no reader. But Zubin Mehta, Philharmonic's new music director, likes to do his own score reading and somehow, in the midst of one of the heaviest conducting schedules around these days, he goes through large numbers of those packages that never stop coming. The process is basically sound. But the wise composers will learn from Makris' suggestions.