Reach out and write someone. That's the message in a new 63-page booklet geared to children. Published by the U.S. Postal Service with assistance from the National Council of Teachers of English, the lavishly illustrated booklet--"P.S. Write Soon"--is aimed at 4th through 8th graders.

Among ideas: ways to make your own stationery, devise secret codes, play chess and checkers in the mail, get a Pen Pal. There are tips on writing fan letters and complaint letters, letters to the editor and to your congressmen. There's even a section on making invisible ink in your kitchen and a list of post office riddles: Why are fishermen good correspondents? Because they're always dropping a line.

To illustrate its point that "anything can happen" when you write a letter, there are copies of an exchange between 11-year-old Grace Bedell urging Abraham Lincoln to grow a beard, "for your face is so thin" and Lincoln's response that whiskers "would be a piece of silly affectation."

"One reason for the new booklet," says Jeanne O'Neill, Postal Service information officer who oversaw completion of the work, "is that the post office has noticed a rise in recent years in the volume of undeliverable mail--an incredible 44 million letters last year--because of incorrect addressing, while at the same time there has been a drop in the number of personal letters being sent." Purpose of the booklet is to encourage a new generation of letter writers who will know how to get their message across.

In addition, "letter writing is a way of teaching composition," says O'Neill, "it makes you organize your thoughts."

Copies of "P.S. Write Soon" are available from the National Council of Teachers of English, 1111 Kenyon Rd., Urbana, Ill. 61801; $2.50 for a single copy, $1.50 in sets of 20 or more for school use.