It's all over but the buying. With the Halston show in his offices in the Olympic Towers just off Fifth Avenue on Thursday, New York designers wrapped up their presentations of clothes for next spring, the final round following the fashion shows in Milan and Paris. Now the buyers pare the excesses in design and price and place their orders.

"Less is more" is an old saw that applies to the new clothes. Less fabric and fullness will be seen next spring: Clothes have been trimmed to a slimmer fit, and excessive details have been erased to simplify many of the new looks.

Designers have chosen other ways to show off this leaner look for spring:

* A renewed emphasis on belts, occasionally wide and stiff, but more often eased with stretch fabric or an adjustable tie to make them more comfortable.

* Hems held close to the knee. While some designers, particularly Ralph Lauren, Perry Ellis and Gloria Sachs offer women the option of ankle-grazing skirts to wear by day in warm weather, not unlike the proportion of the prairie skirts of last summer, most others opt for a shorter knee length.

* Last year's winning stripes have been dashed for dots. Usually they are simple, evenly scattered dots, but once in a while they become tricky and even look like amoeba or jellyfish. Occasionally, buttons are so big they look like huge dots.

* White is the obvious successor to all the black clothes of this winter. Dots with the most dash are black on white or the reverse, and several designers have chosen to bisect clothes in black and white as well. Bright colors are the occasional alternative to black and white.

* With clothes more fitted to the body, rear view has become as important as the front. For that reason designers have put buttons down the backs of skirts, slit hems up the back rather than the front and bared the back in many ways.

* More emphasis on evening clothes with the expectation that women will spend money on special-occasion than everyday clothes. Strapless and one-shoulder designs show up often in glitzy fabrics. Ruffles are still around for evening, but the best evening clothes by far are the long lean ones in rich fabrics.

Harriet Winter, whose label reads Mrs. H. Winter, is one of the few designers who finds ways to cut clothes and still keep the style of clean, easy shapes she has always liked. This year she has narrowed her silhouette with darts used horizontally to pull in clothes at the waist, and channel seaming and cartridge pleating to fit the clothes closer to the body.

Winter sat on a stool and talked about her clothes, adjusting them on models as they passed by. All the models wore fabric shoes--"leather is simply prohibitive," she said. Her wedding dress was plastic, meant to be worn also as a raincoat, she said.

One designer concerned with making the price tags leaner as well as the clothes is Adri, this year's winner of the Coty Award, the fashion industry's top prize. "I think the customer is sick of buying designer sportswear at close to $200 per item and reaching $600 to $900 per outfit," she said.

Adri has decided to do something about it--at least with part of her collection. She has cut about 25 percent of the final price by being more selective about fabrics and producing clothes in the Far East. "A couple of years ago no one was concerned about price," she said. "Now, you can pass up a $23-a-yard fabric and use one for $14 a yard, and it is hard to tell the difference between the two." Occasionally she uses a mix of linen and rayon instead of all linen, which might reduce the price by 15 percent, "and also makes the garment wrinkle less."

She is switching much of her production to the Orient, she says, for both better workmanship and lower prices. "It means that we have to produce a greater quantity of each item, but that makes us choose each item we make more carefully," said Adri, who figures that many of her styles can retail for $30 less in her spring collection than they did in the fall.

That may be the best fashion news this season.