Since Rubens' rounded bellies went out of style, men and women have struggled with girth control -- mostly unsuccessfully. Behind the success stories, however, lie discoveries of one sort or another, and Jim Everroad's story began five years ago when he discovered his potbelly.

In case you've never heard of him, Jim Everroad wrote "How to Flatten Your Stomach," the best-selling $1.75 pamphlet (over 1 1/2 years on the trade paperback bestseller list and more than a year on The New York Times list).

But back to his stomach. The bulge was hardly a startling discovery for Everroad, then in his early 30s, but it tended to undermine "my professional image" as high-school gym coach and ex-gymnast. Still, he carried his pot around for four or five years before deciding it had to go.

"I started," he said from his home in Columbus, Ind., "by taking the exercises I liked best from my athletic training over the years. I went back to my old anatomy and physiology books and worked out a program to tighten up the stomach muscles. It worked.

"At first, my family thought I was kind of crazy. That summer of '74 when I was trying to put the program together, I'd get up at 6 in the morning and put on a shirt and tie so I could feel businesslike. Then I'd go settle down in my office, which was a corner of the bedroom, and sit and study.

"I had a friend who was a serious runner -- this was before running was a fad -- and every morning, he'd stop by on his way back from running and make wisecracks about my office/bedroom and my crazy plan.

"I'd keep studying till noon or 1 in the afternoon, then I'd go exercise and finally head for the community pool and lie around there till about 3 or 4 o'clock. My wife thought I was off the track a little. She wanted the house painted that summer. Well, she painted most of it herself. No, she wasn't too pleased with me that summer."

Everroad wanted to write a book about his success, but wasn't sure how to go about it.

"I was looking for something different to do. I'd been teaching for eight years, a long time. "Then I began talking to a sportswriter at The Gary (Ind.) Post Tribune. He offered to give me an article with pictures (of Everroad and the daughter of a Columbus hardware store owner demonstrating exercises)."

The response was a whopping 500 requests for details of the program. Everroad had 3,000 copies of his exercise leaflet printed and sent them out at $1 a copy, postage and handling included.

"At that point," says Everroad, "I thought I was in publishing."

He decided to branch out into the mail-order business through newspaper advertising. The next summer, he wrote letters of inquiry to about 200 newspapers. He also quit his job at a high school in Crown Point, Ind., "my first strategic blunder."

"That fall," he says, "I ran about $2,000 worth of advertising in one week -- and went broke."

The only steady sales, a couple dozen copies at a time, were to a nearby B. Dalton bookstore. The modest demand continued for about three years, a thousand copies' worth. Meanwhile, Everroad had kept the letters from B. Dalton requesting more and more copies. He put the letters together to use in another mail-order campaign, for which he printed an additional 20,000 copies. He managed to sell about 5,000 of those.

Encouraged, he quit another job (at a health spa in Columbus), loaded up his VW with the remaining 15,000 copies, and set out across Ohio, dropping off books on consignment. It took him eight weeks to go broke again.

The big breakthrough came about three months later: Price/Stern/Sloan, a small California publisher of how-to paperbacks, offered to buy his book and also hired him to sell it in the Midwest.

That version of the 28-page, illustrated paperback appeared in late 1977. The bulk of the first 20,000 copies was sold to B. Dalton, where the book made its trade paperback bestseller list in January 1978. Five months later, it hit The New York Times bestseller list and it's been there since. To date, 900,000 copies have been sold.

"My kids," says Everroad, "don't quite understand what I'm doing. Sometimes I don't either. Once, my 6-year-old daughter said 'You sure are a funny daddy.'

"But they enjoy all the celebrity stuff. When I was on a TV show with F. Lee Bailey I took my 11-year-old daughter along. She didn't know who he was. I think it was deflating for him.

"Another time, she met Sandy Dennis in Cincinnati. She didn't know she had won an Academy Award, but she learned that Sandy had 50 cats and got her autograph. She was thrilled."

Everroad, now 37, has come out with a sequel: "How to Trim Your Hips and Shape Your Thighs," another $1.75 bestseller, written in collaboration with Lonna Mosow, who does a physical-fitness show for a Minneapolis television station.

"It's the only time in history," says Price/Stern/Sloan representative Julie Nathanson, "when the same author has had two books on the trade paperback bestseller list simultaneously."

Coach Everroad admits after some prompting: "My average combined salary from teaching senior high school and coaching was $9,500 a year. This year, royalties from the stomach book will be more than all the money I made from nine years of teaching and 10 years of coaching."

And he has a stomach so concave it ought to go on permanent display at the Museum of Modern Art.