Traditional dinner-party decor -- white linen tablecloth, matching napkins and large floral centerpiece -- has gone the way of white gloves, maintains White House Social Secretary Gretchen Position.

"We've moved away from those very rigid decorating guidelines we were brought up with," says Poston, who has used everything from toy cars to sand and seashells as centerpieces for White Houe dinners and has ordered tablescloths in every imaginable color and pattern.

"Occasionally we'll go back to the white linen tablecloth, but usually we're more free. I don't find I'm any less correct.It's just that we've found it makes things more interesting and fun."

A gracious, unflappable woman who spent a half-dozen years in wedding and convention planning before joining the First Lady's staff, Poston's forte is imagination. She sees herself almost as "an extension of Mrs. Carter" whose photograph, along with Amy's, adorns her office wall.

"As any hostess, Mrs. Carter wants to please her visitors," says Poston. Her secret for creating a successful party -- from a state dinner to a backyard barbecue -- is "relating the table to the people." a

As soon as Poston is notified about an upcoming state dinner -- anywhere from a month in advance to just four days away as happened with the peace treaty clebration -- she visits the country's ambassador.

"We talk about the people who are coming -- what they like and dislike." Then Poston tries to select decorations and entertainment according to the guests' personal interests.

"For example, the (West German) ambassador told me that when Mrs. (Helmut) Schmidt, who is a horticulturist, was here during the Bicentennial she fell in love with the crape myrtle that grows wild here.

"We had the house filled with them, and decorated the tables with a miniature garden centerpiece of crape myrtle. Well, she walked in, took one look and she cried."

After Poston discovered that Japanese Prime Minister Masayoshi Ohira had always wanted to attend an American barbeque, the state dinner in his honor was planned for the West Terrace with buffalo meat roasting on barbecue drums. Each table, set with a green and white ruffled tablecloth, boasted a clay pot filled with spring blooming plants surrounded by hurricane candles.

Poston is making a light exception to her "relating-the-table-to-the-people" rule for Monday's state dinner honoring British Prime Minister Margaret Thatcher.

Since the dinner falls so close to Christmas, Poston has selected decorations to complement the White House Christmas tree, which is decked with handmade ornaments representing 200years of American folk art.

"We wanted it to be seasonal, yet not the trite red and green," says Poston.

The chocie: lavender quilted tablecloths and napkins with Christmas-red stitching and vermeil bowls filled with red, purple and white anemone blossoms. As a Christmas remembrance, handmade dried flower nosegays will be set at each woman's place.

Poston says she never runs out of new ideas because "if you're interested enough to find out about the people and their likes, the ideas will come. We could just put out a cloth and a bouquet of flowers -- who's going to complain about the White House?

"But it's so much fun to relate it to the evening and the people. And a Dinner Party is a Dinner Party. You don't do it unless you do it right."

One of the most common mistakes in decorating for a party, says Poston, is "not being true to yourself. You shouldn't pretend you're the richest person in the world and use sterling and 100 waiters.

"You should do what works with your home and personality and can be done well. It's the thought and preparation that can make the party.

"You don't have to spend a lot of money. Candles are the secret; they can do wonderful things for a house in the evening. You can go to Murphys and buy pretty little plants, use branches from trees in your yard or have one perfect flower.

"You can serve chili and make things fun and festive, with bright colors and ceramic decorations. I'd rather have one party people remember than 10 hastily, not well-done parties."

Another big mistake is "when the hostess keeps jumping up and down. She's so distracted that the guests are uncomfortable. Have someone else serve, whether it's your kids or a professional."

Another error is overcrowding. "Guests should be made to fell that they're important, and they won't feel that way if they're squished in."

While Poston beleives in the "pulled-together look" for an event, she shies away from the idea of a "theme party."

"Theme parties work on occasion. You may want to have a luau in your backyard. But a theme party tends to exaggerate and package an idea, which was popular in the '50s, or when we were 16.

"What you're looking for now is to have everything so quiet and pulled together that it works. And you do want to look for touches that will be individually geared to your guests.

"We're into a more understated, quiet more elegant time. Nothing should be standing out so that it's garish. It should be a total feeling, that extends to each room the guests are in.

"And everyone has an art object or something around their house that they're proud of. Put it on the table with candles or alone as a focal point."

What if a favorite object is broken when someone reaches for the salt?

"Life is too short to be brothered by that," shrugs Poston (the mother of four). "Everything in my house has been glued. You can use things or keep them hidden. I feel if I like something, it should be out for everyone to enjoy."