PARIS-Yves Saint Laurent proved two things yesterday with his standout collection of ready-to-wear for fall. When he bounded out on stage and waved to the cheering audience of buyers and onlookers, he put down persistent rumors that he was sick or had died. He was, in fact, trimmer than at the collections six months ago, and conservatively dressed in a suit and tie.

And the 180-plus piece collection showed that he has endless ideas for refreshing familiar clothes as well as suggesting new ones. "He's the most prolific designer in the world," said an enthusiastic Robert Sakowitz, president of the Texas-based stores of the same name after the show.

Saint Laurent's strength, perhaps his genius, is to design clothes that are suitable for the way his customers live and to make them so attractive they're hard to pass up. Partly it's a sense of timing, but he also has the eye to refine prevailing trends with fabric and proportion and to interject a few new ideas along the way.

So what will Saint Laurent customer'swear next fall? And what will numerous other designers copy to provide the YSL look at cheaper prices?

The Victorian nightgown look with ruffed neckline and a ruffle to outline the yoke at the top of the blouse, dress or coat.

The tiered skirt, carried forth from earlier collections in printed wool challis for day, other fabrics including a bantam-weight lame mousseline for evening.

Pants of all lengths, including a ribbed knit style shaped like sweat pants with cuffs or tucked into boots, and paired with big sweaters or blouson tops.

Black for evening-a black velvet suit with Pierrot blouse, or black and gold chiffon in many patterns for full sleeved blouses and skirts wrapped up with a big shawl and gold sandals or gold boots.

Fringe used to soften the edges of coats, blouses and dresses but also trimmings boots, bags and of course shawls.

For the first time in three seasons Saint Laurent chose to send his models down the runway without five models striding forth at one time to make a forceful statement. The presentation was far more conservative and so, in fact, were the clothes. The gimmickry is gone and good saleable fashion has replaced it.

"Yves is feeling more conservative about everything," said Gustave Zumsteg of the House of Abraham, the Swiss firm that develops most of the fabrics Saint Laurent uses. And since the shapes are not dramatically new, it's the fabrics, the work of Saint Laurent and Zumsteg together, that sets the collection apart.

According to Zumsteg, the paisley pattern that predominates was tested by Saint Laurent in his spring couture collection. Zumsteg started with old documents, but made the paisleys for the collection in larger proportions and in paler fabrics to make them look contemporary. They show up as successfully in lightweight wool challis as in a Victorian-style chemise in paisley printed silk mouseline worn over a very full skirt, and in scarves of cashmere and silk, occasionally with a little glitter mixed in.

Most of Saint Laurent's hemlines drop way below the knee. Hemlines that stop at the knee were apparently an afterthought, added, it is said, "to get a fresh look." They are usually worn with Mary Jane style black patent shoes with grosgrain bows, a nearly flat wedge and black stockings. These are a welcome change from the tarty looking stiletto heels shown with many of the shorter skirts these days in Paris.

Never one to abandon his faithful clients who part with sizeable sums each season, Saint Laurent repeats some familiar themes, often updating them. The piped coat is still around but there is a new version that is shorter with a fuller peplum (flared skirt). Zoave (knee-length harem) pants are in the collection but adapted to the look of a clown rather than a desert soldier. His baby doll dresses with velvet empire bodices and puffed sleeves have grown up to classier versions in wool challis and velvet, but they hardly make the old style look passe.

Those who liked taffeta last season can indulge in more conservative skirts of taffeta, this time in moire. Capes are still around so your old one need not go to Goodwill. The new one is a steal from the Tunisian shepherds with a tasseled hood.

And for the corselet aficionado, there is a tight-fitting black velvet jacket that fits like a corselet but is very conservative with big puffed taffeta sleeves.