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 — 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."