"In the theater," says designer Halston, "your clothes become your dancing partner. And they can give you additional movement." So when he created the costumes for his friend Liza Minelli for "Liza . . . In Concert," which opens Nov. 13 at the Kennedy Center, they worked together for days as he watched her routines, then tried out the clothes as she ran through her act. "In theater you have to workk around what the performer is doing," he added. "When she raises her arm, the jacket must stay down, the halter can't be vulnerable and the pants can't rip on a high kick."

Halston has done more than just Liza's costumes for the performance, which started its run in New York. He's put the orchestra in black shirts and black pants "so you see only faces and instruments, not pot bellies and an occasional unbuttoned jacket." (Halston also did the costumes for the musicians when he costumed Sly and The Family Stone.) The dancers, whom he has dressed as well, start out looking like band members, but change to white suits, with Liza in a white bugle-beaded fringed dress.

Liza has six major changes plus a cabaret costume, and most changes are made on stage. Because of the hardwear of 200 performances Halston has made two of each outfit.

It may seem like a simple everyday thing, but to psychologist Joyce Brothers a child's choice of clothes can affect that child's personality development. Brothers, a consultant to Garanimals, a line of childrens sportswear, says selecting clothes is one of the few things you can leave totally up to a child. "You cant't tell him or her to decide when to go to bed . . . they will never admit to being tired. You can't let them decide when to cross the street (if they are very young) because of the dire consequences."

But, says Brothers, you can let them decide how to spend a small allowance.

If they need to borrow money be sure to charge interest, she warns. You can let them choose toys and decide if they were misled by a television ad. With clothes, she adds, a child can also learn. If friends like what they wear, or if a friend says "yuck" it will affect their choices next time. "The kind of decision making where there is playback is what builds confidence," she says. "The more you succeed, the more you are willing to try something new. When you can decide something with favorable results, such successes build confidence."

Brothers believes that if you let your child dress in the manner of the ingroup as a youngster, "they will have less need later to push their way into an in-group by doing dangerous things like smoking, drinking or taking drugs.

Brothers, who plotted animal-shaped hangtags for Garanimals that help kids select matching tops and bottoms, exexpects to have a hand in the design of the garments before long. (Garanimals are carried by Hecht's.)

What's the most expensive costume ever made for a movie extra. If you guessed the swathed-in-sable at a swish party, guess again. It was probably the costume created by Robert Fletcher for Betelgeusian Chief Ambassador, 23rd century alien in Paramount's "Star Trek". According to Fletcher, the fabric alone had a price tag of $10,000.

The fabric was (real) silver and (real) gold brocade woven with designs of leopards and falcons. It took 12 yards to make the costume.

Fletcher worked for a varied costume look for the aliens. "I, for one, hate to see the whole earth dressed exactly alike. And I think there is a trend today among Eastern and Mideastern people to reject Western-style clothing. That's healthy," he says."I love to see the burnous, the djellaba, the chadoor, the caftan and all the rest."

Guess who's riding high on Western wear? Sammy Davis Jr., master of gold and glitter, has switched to Western, at least for the moment. In spite of the disappointing response to the big Western garb push in fashion so far this fall, Davis asked to be on the cover of the Ah Men catalogue from the West Hollywood mens shop. According to Don Cook, the shop's owner, Davis did the cover gratis.

Mister Glitz himself, Liberace, admits that even for him, all of the lavishly embroidered clothes have gotten too expensive, "But as long as audiences get a kick out of them I'll keep on trying to top myself." So in his recent show at the Celebrity Room at John Ascuaga's Nugget he donned a cape of Norwegian blue shadow fox, insured at $300,000. The cape, modelled after a coronation robe with a train 12 feet wide and 16 feet long "is the most expensive fur piece ever created," says Anna Nateece, former designer to Queen Frederika of Greece. The cape alternates jeweled, silk brocade with fur and, of course, the brocade also works as lining.

It was a store buyers dream. Cornelia Noland, co-owner of Nuevo Mondo was on a buying trip in India and took time out to rest on a houseboat in Kashmir. Who pulled up alongside on a raft, but a man selling embroidered cashmere jackets. "They take two months to embroider," said Noland showing off some of the styles in her Alexandria shop, "and they are a treasure a woman will own for a lifetime."