NEW YORK - As a motivational speaker and executive coach, Caroline Adams Miller knows a few things about using mental exercises to achieve goals. But last year, one exercise she was asked to try took her by surprise.

Every night, she was to think of three good things that happened that day and analyze why they occurred. That was supposed to increase her overall happiness.

"I thought it was too simple to be effective," said Miller, 44, of Bethesda. Md. "I went to Harvard. I'm used to things being complicated."

Miller was assigned the task as homework in a master's degree program. But as a chronic worrier, she knew she could use the kind of boost the exercise was supposed to deliver.

She got it.

"The quality of my dreams has changed, I never have trouble falling asleep and I do feel happier," she said.

Results may vary, as they say in the weight-loss ads. But that exercise is one of several that have shown preliminary promise in recent research into how people can make themselves happier ¡ª not just for a day or two, but long-term. It's part of a larger body of work that challenges a long-standing skepticism about whether that's even possible.

There's no shortage of advice in how to become a happier person, as a visit to any bookstore will demonstrate. In fact, Martin Seligman of the University of Pennsylvania and colleagues have collected more than 100 specific recommendations, ranging from those of the Buddha through the self-improvement industry of the 1990s.

The problem is, most of the books on store shelves aren't backed up by rigorous research, says Sonja Lyubomirsky, a psychologist at the University of California, Riverside, who's conducting such studies now. (She's also writing her own book).

In fact, she says, there has been very little research in how people become happier.

Why? The big reason, she said, is that many researchers have considered that quest to be futile.

For decades, a widely accepted view has been that people are stuck with a basic setting on their happiness thermostat. It says the effects of good or bad life events like marriage, a raise, divorce, or disability will simply fade with time.

We adapt to them just like we stop noticing a bad odor from behind the living room couch after a while, this theory says. So this adaptation would seem to doom any deliberate attempt to raise a person's basic happiness setting.

As two researchers put it in 1996, "It may be that trying to be happier is as futile as trying to be taller."

But recent long-term studies have revealed that the happiness thermostat is more malleable than the popular theory maintained, at least in its extreme form. "Set-point is not destiny," says psychologist Ed Diener of the University of Illinois.

One new study showing change in happiness levels followed thousands of Germans for 17 years. It found that about a quarter changed significantly over that time in their basic level of satisfaction with life. (That's a popular happiness measure; some studies sample how one feels through the day instead.) Nearly a tenth of the German participants changed by three points or more on a 10-point scale.

Other studies show an effect of specific life events, though of course the results are averages and can't predict what will happen to particular individuals. Results show long-lasting shadows associated with events like serious disability, divorce, widowhood, and getting laid off.

The boost from getting married, on the other hand, seems to dissipate after about two years, says psychologist Richard E. Lucas of Michigan State University.

What about the joys of having children? Parents recall those years with fondness, but studies show childrearing takes a toll on marital satisfaction, Harvard psychologist Daniel Gilbert notes in his recent book, "Stumbling on Happiness." Parents gain in satisfaction as their kids leave home, he said.

"Despite what we read in the popular press," he writes, "the only known symptom of 'empty nest syndrome' is increased smiling."

Gilbert says people are awful at predicting what will make them happy. Yet, Lucas says, "most people are happy most of the time." That is, in a group of people who have reasonably good health and income, most will probably rate a 7.5 or so on a happiness scale of zero to 10, he says.

Still, many people want to be happier. What can they do? That's where research by Lyubomirsky, Seligman and others comes in.

Caroline Adams Miller, a motivational speaker, addresses a group of young competitive swimmers at American University about 'happiness' Thursday, Nov. 9, 2006, in northwest Washington. [AP]

The think-of-three-good-things exercise that Miller, the motivational speaker, found so simplistic at first is among those being tested by Seligman's group at the University of Pennsylvania.

People keep doing it on their own because it's immediately rewarding, said Seligman colleague Acacia Parks. It makes people focus more on good things that happen, which might otherwise be forgotten because of daily disappointments, she said.

Miller said the exercise made her notice more good things in her day, and that now she routinely lists 10 or 20 of them rather than just three.

A second approach that has shown promise in Seligman's group has people discover their personal strengths through a specialized questionnaire and choose the five most prominent ones. Then, every day for a week, they are to apply one or more of their strengths in a new way.

Strengths include things like the ability to find humor or summon enthusiasm, appreciation of beauty, curiosity and love of learning. The idea of the exercise is that using one's major "signature" strengths may be a good way to get engaged in satisfying activities.

These two exercises were among five tested on more than 500 people who'd visited a Web site called "Authentic Happiness." Seligman and colleagues reported last year that the two exercises increased happiness and reduced depressive symptoms for the six months that researchers tracked the participants. The effect was greater for people who kept doing the exercises frequently. A followup study has recently begun.

Another approach under study now is having people work on savoring the pleasing things in their lives like a warm shower or a good breakfast, Parks said. Yet another promising approach is having people write down what they want to be remembered for, to help them bring their daily activities in line with what's really important to them, she said.

Lyubomirsky, meanwhile, is testing some other simple strategies. "This is not rocket science," she said.

For example, in one experiment, participants were asked to regularly practice random acts of kindness, things like holding a door open for a stranger or doing a roommate's dishes, for 10 weeks. The idea was to improve a person's self-image and promote good interactions with other people.

Participants who performed a variety of acts, rather than repeating the same ones, showed an increase in happiness even a month after the experiment was concluded. Those who kept on doing the acts on their own did better than those who didn't.

Other approaches she has found some preliminary promise for include thinking about the happiest day in your life over and over again, without analyzing it, and writing about how you'll be 10 years from now, assuming everything goes just right.

Some strategies appear to work better for some people than others, so it's important to get the right fit, she said.

But it'll take more work to see just how long the happiness boost from all these interventions actually lasts, with studies tracking people for many months or years, Lyubomirsky said.

Any long-term effect will probably depend on people continuing to work at it, just as folks who move to southern California can lose their appreciation of the ocean and weather unless they pursue activities that highlight those natural benefits, she said.

In fact, Diener says, happiness probably is really about work and striving.

"Happiness is the process, not the place," he said via e-mail. "So many of us think that when we get everything just right, and obtain certain goals and circumstances, everything will be in place and we will be happy.... But once we get everything in place, we still need new goals and activities. The Princess could not just stop when she got the Prince."