Has it really been more than two months since Day 1 of Diet 1991? You lost that first fast five pounds, then another two, then one, now nothing.

Diet die-hards call it "hitting the plateau." A little snack here, a little brownie there, and you're right back where you started.Nearly one in three Americans is on a diet, about half of the women and 25 percent of the men.

Long-term, most of them will fail to keep off the weight. According to some estimates, only an abysmal 2 percent to 5 percent keep off lost pounds for at least two years. Repeat business is why there's a $39 billion diet industry.

Whether the dieter is paying for advice or getting it free, experts agree on one thing. There are only three ways to lose weight:

Eat less. Exercise more. Do both.

The last is most effective, according to a dietitian, a nutritionist and an image consultant/aerobics teacher. They hope to help people part company with yo-yo dieting.

One disclaimer: Any diet plan, diet powder, diet program has its successes - people who lost the weight and kept it off. This story is for people who've tried everything, who want to change their relationship with food.

If you've ever hit the plateau, you understand these dieters who think they're doing all the right things . . .

- I FOLLOWED THE PROGRAM faithfully, but . . .

Diet programs are the problem, not the solution, said Shirley Billigmeier of Minneapolis, whose book "Inner Eating: How To Free Yourself Forever From the Tyranny of Food" recently arrived in bookstores. She developed her theory because "I wanted to be free of thinking about food all the time."

Billigmeier said rigid diet programs or diet foods put the dieter inside a box - all the food inside the box is "good" food; everything outside is "bad."

"And if you eat it, you're bad, too," she said during a recent book tour. Once dieters veer outside the safe little box, if they eat one m & m, they might as well finish the bag.

Inner eating is to "take ownership of what you're eating, listen to your body," eat only when you're hungry and don't let somebody else make your choices for you.

- I FOLLOWED THE PROGRAM, but . . . Part II.

The more rigid the program, the more the dieter thinks she can control what she's eating, but the opposite is true, said Dr. Colleen Sundermeyer of Sacramento, Calif., author of "Emotional Weight" and "Eat No Evil," out next month.

"People are suffocated by that in the long run. It takes all my willpower not to grab them away from the SlimFast and say, `Behave yourself.' Businesses want to make money so they create these diet regimens."

- I COMPLETELY ELIMINATED FAT from my diet, but . . .

You probably feel hungry all the time. Some fat is needed in a healthy diet, said Pat Harper, a registered dietitian and a spokeswoman for the American Dietetic Association.

Fat provides satiety, the feeling of satisfaction.

Still, reducing fats and oils can help you lose weight - an ounce of fat has twice the calories of an ounce of carbohydrates or an ounce of protein. Moderation is the key.

- MY WEIGHT BOUNCES around from day to day, and . . .

Throw out the scale, said Sundermeyer, who earned her master's degree in psychology and her doctorate in nutrition. "Feel your own body," she said. "We're used to getting approval from the scale." Approval should come from within.

Keep scales away from teenage girls, who are often obsessive about asingle pound. "They were happy 21/2 seconds ago. Now they're not."

- I EXERCISE LIKE CRAZY, but . . .

Obsessive exercise may have the same poor effect as no exercise at all. When the body feels threatened by high energy use with few calories coming in, it reacts by holding on to the food eaten.

"When I say exercise, people think of punishment - they're exercising in response to eating that Hostess Twinkie," said Sundermeyer. "We are born with every mechanism not to overeat, but we eat in response to our emotions, from being too busy to stop and acknowledge our bodies."

- I HARDLY EAT anything, but . . .

Do you skip breakfast? Miss lunch? That's the worst thing a dieter can do. If the body is deprived of food, the metabolism slows to avoid starvation.

"When people reach that plateau, or set point, that's the body trying to reach equilibrium," said Sundermeyer. "The best way is to lose weight slowly coupled with regular exercise."

- I HAVEN'T HAD A TREAT in three weeks, but . . .

Eliminate everything you like from your diet and a binge may be on the way. "I insist people plan treats," said Harper.

The dieter thinks about chips, dreams about chips and it may be better just to eat some, as long as it's "active eating," said Billigmeier.

According to her, the diet industry says, "Give us your money, and we'll tell you what to eat," which is self-defeating.

- I ONLY HAD ONE bran muffin, but . . .

But get a gander at that giant - it's really three muffins.

Portions are a boondoggle with many dieters, said Harper. People see a TV dinner as not much food, but those are standard portions. "Those servings you get in the cafeteria line - like a half-cup of applesauce? Well, that's a serving."

- I'M BUYING ONLY DIET FOOD, but . . .

There are two trains of thought on whether "diet" foods are a good idea. One tenet holds that it's best to cut anyplace you can, then choose how to work in favorite foods.

It's not individual foods, but the diet as a whole that causes overweight, said Harper, who counsels on losing weight. "Cut all the painless things. Use the vegetable spray, choose the leanest meats, then along comes the double fudge whipped cream nut torte . . . Make all the easy changes - the hard ones you weigh as you go."

The other option is to forget specialty foods and reduce portions of regular foods.

A middle ground might be Sundermeyer's plan to save calories in foods you cook yourself. She described the "flavor point," in which food doesn't gain flavor by adding more fat or other rich ingredients. Use a bit of butter, then substitute for the rest; use skim milk in sauces.

In her upcoming book she listed two pages of substitutions with the recipes. That may require more cooking. "Get back in the kitchen. Take responsibility for what you're putting in your body."

- I'VE BEEN FOLLOWING the diet for nine weeks, but . . .

You're probably incredibly bored. Any day now, you may give up.

"The human body craves variety," said Billigmeier, who has a master's degree in physical education and health. She told of a client who would eat a whole pan of brownies at a sitting. If she had decided to have a brownie every day, she probably would have soon grown tired of them. "Food isn't a drug. With food, there is a satisfaction point," said Billigmeier.

Decide how certain foods make your body feel. Energetic? Lethargic? If you're not hungry, don't eat.

"I once asked my mother what it felt like to be hungry," she recalled. "She looked at me real funny and said, `I don't think I've ever experienced hunger.' "

In contrast, her husband never seems to think about food and doesn't worry about going to his closet because his clothes fit. Recognize where the good taste ended, when satisfaction set in, when to stop.

Billigmeier said she recently ordered a chicken breast in a restaurant. "When I was always on a diet, I couldn't have faced another chicken breast," she said.

Twenty pounds lighter and never on a diet, she compared inner eating with playing tennis. When you're doing it right, you're not thinking about the ball. The same goes for food.