He strides into a room the very image of authority: his figure tall, slender and ramrod straight, his face rugged, his hair thick and gray. He's dressed in his "power" outfit.That's right, power.

No, not blue tights with a big red "S" emblazoned across the chest, but a conservatively cut navy-blue flannel suit and vest, with pin-dot navy silk tie, plain white shirt and pocket hankerchief, knee-length black socks and black wing-tip shoes.

Clothing with the power to impress. To influence. To intimidate.

"Walk into a nice restaurant dressed like the average individual and you'll be given average service. Look like you and your family have had money since Day One and You'll be treated accordingly," says men's fashion adviser William Thourlby.

Years ago, Thourlby learned about the way clothes can change a person's image. Fellow actors would show up on the set of such TV series as "Rawhide," "Wyatt Earp" and "Perry Mason, some "well-dressed, some funny and some looking just ridiculous."

But, "They would go into wardrobe and come out looking like men of authority" -- like judges, lawyers, doctors, scientists. "I thought, 'Here is a science that should be out in the street.'"

Eventually he gave up acting -- after a two-year Braodway stint with Jayne Mansfield in "Will Success Spoil Rock Hunter" -- to do just that. He now flies about the country advising aspiring males (at $50 an hour) on how to dress for the executive suite.

A mont ago, he says, a $250,000-a-year bigwig sought his help. The kind of man who has pictures of himself with LBJ, Kennedy, Prime Minister Gandhi. He had his chaufferu meet Thourlby at the airport.

"I want to look the best, the top man, a man of authority and a man of wealth," he told Thourlby.

"He took me down to his bedroom and I looked at his clothing," says Thourlby. "I threw out 38 suits."

The executive, he says, "was going all over the world wearing scarlet and polka-dot linings in his jackets, cloth belts, double-breasted vests, pearl and brass buttons, and tricky color combinations." Tailored and expensive, yes.

"Everything, but right."

Thourlby told the man to give up his diamond rings, and a digital wristwatch "that had to be $2000. It looked all wrong for what he wanted to say."

Thourlby took the man on a three-day shopping spree, and afterward -- in much-subdued attire -- the man admitted about his other wardrobe: "I knew in London when they didn't invite me in, something was wrong."

Thourlby calls himself a "wardrobe architect." Operating out of Manhattan, he lectures on proper dress to up-ward-bound employes of such establishment firms as Coca-Cola and Price Waterhouse. Often he carries along a gaudy tie and bright sports jacket to illustrate the "Dont's" (He frequently spots the same jacket-tie combination in the audience.)

He has put his advice in a book, "You Are What You Wear," just out in a Signet paperback (176 pages, $2.25). It's a play-it-safe guide to the man who wants to "package himself for success" in his career, or for fun. Stick to the classic-cut black and navy suits on the job, he advises; save the body shirts and hip-hugging trousers that women find appealing for after hours.

The Carter people once phoned him for his reaction, he says, after the president wore a cardigan sweater for a fireside chat.

"Everybody," he told them, "knows the president of the greatest nation in the world wore a cardigan sweater.Nobody knows what he said."

That's the last time the White House asked his advice.

Thourlby looks dapper and distinguished -- correct, if somewhat lacking in flair -- in his executive dress. That alone might catch your attention. But there's something else.

"I was the first Marlboro man."

That cigarette ad campaign, he says, had a big impact on how males are perceived in this country. Before, "I was considered too rough looking." But overnight, craggy-faced men like himself "became attractive." It was "a new era." Pretty boys such as Robert Taylor and Tyrone Power were out and the rougher-visaged Kirk Douglas and Burt Lancaster were in.

Thourlby speaks and writes with the earnestness of a crusader out to save the American male.

"When you step into a room," he writes, "even though no one in the room knows you or has seen you before, they will make 10 decisions about you based solely on your appearance." These include "your economic level, educational level, trustworthiness, social position, level of sophistication, economic heritage, social heritage, educational heritage, your success" and "your moral character.

"If your feel this isn't fair," he continues, "that a person should be judged, not by what he wears but by what kind of person he is and by what he has done with his life, remember this: Life listens only to winners."

Thourlby's horror tales -- promising employes passed over for promotion because of double-knit suits, bright shirts, white belts and a scruffy beard -- could leave you weeping over your own wardrobe.

"Believe me, no one is ever promoted to an executive position if he would make the other executives uncomfortable by his appearance."

Thourlby, who considers himself only a No.5 in looks, says, "You don't have to have perfect lips or nose." Tough-guy actor Edward G. Robinson could hardly be called handsome, "but the way he was packaged, the way he walked, he was a man of authority."

There are no "unattractive men," says Thourlby, "only lazy ones."