If the store buyers rarely clapped until the end of the Yves Saint Laurent show yesterday, it was only because they were busy counting all the money they would make from this familiar collection of classically tailored clothes.

The Saint Laurent show, in fact, was typical of this season -- which has been much applauded by store buyers for making sales rather than news. Like most of the designers this week, St. Laurent has refined the shapes of seasons past with new fabrics and clearer colors.

At the best shows, specifically Ungaro, Lagerfeld for Chloe, Givenchy and Claude Montana, the collections have been developed with the designer's signature, yet without the exaggerations of other seasons.

The big news is the appearance of the long skirt, though there are short ones as well. There are voluminous shapes, often controlled with belts, but also many slim shapes. While many of the designers like the look of lightweight layers, particularly skirts over pants, others are paired down to simpler parts.

"It is not the old layering," insists Karl Lagerfeld. "It is weightless volume. Volume for movement, not for weight." He has put long, military school-coat dresses over ankle-length cuffed pants. France Andrevie, Sonia Rykiel and Kenzo have also teamed up this new proportion.

"I tried it on this morning, and it looks and feels wonderful," said I. Magnin's fashion director, Sonia Caproni. "It is like wearing a very light coat dress instead of a jacket with skirt and trousers.

Wearing a skirt over pants may become a new style around the White House, says Geraldine Stutz, head of Henri Bendel. "I bet Nancy Reagan would love pants worn with slit skirts over them. You end up with skirted pants that look very modern and new."

Saint Laurent, with his knee-length skirts and ladylike shapes, really knows the Washington woman, insists Val Cook of Saks-Jandel, who particularly likes his evening collection. "I think he took his cue for the season from the White House."

(YSL's understanding of the customer apparently goes beyond the runway. In the middle of the night before his show he had some workmen fill in the potholes in the street outside the tents.)

If YSL has passed on some of the oversized looks of the season, he has not shortchanged his customers with new ideas. They will find new plaid wool voile tops, a new three-quarter coat silhouette, a variation on the blouson, hankerchief-hem ruffled dance dresses, and floor-length skirts for evening.

Saint Laurent has used a lot of black again for evening, particularly black velvet. He often brightens with gold accessories and shoes. And while he started the gold rush and is all over the streets and runways of Paris, he uses gold far more subtly in prints for blouses, evening dresses and shawls. h

Concludes Gerladine Stutz, "Thanks to Saint Laurent, every party is going to look like a visit to Fort Know this season."