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Anyone who has tried to lose weight knows that trimming pounds is the easy (or easier) part. Keeping them off is the challenge, as boredom, tempting food and sedentary living erode your resolve.

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Yet surprisingly few studies have examined how best to maintain weight loss, leaving a missing piece in the anti-obesity puzzle.

Now a Brown Medical School study published last week in the New England Journal of Medicine suggests that the bathroom scale, an emergency diet toolbox and cues from a stoplight might hold the keys to success.

The study took a lesson from the National Weight Control Registry, a group of more than 5,000 "successful losers" who have shed at least 30 pounds and kept them off for at least a year. Registry members have trimmed their waistlines in a variety of ways, from cutting calories and boosting exercise on their own to joining groups such as Weight Watchers.

One habit they share: regular weigh-ins and then adjustment of food and exercise when pounds start to creep back on. (Successful losers also rarely miss breakfast and get at least an hour a day of physical activity, such as brisk walking.)

But can their strategies help others maintain their reduced weight? That's the question tested by the new study, which involved 314 people who had lost at least 10 percent of their body weight during the previous two years.

All participants received a bathroom scale along with instructions to use it often. They were encouraged to report their weight weekly by telephone.

During the 18-month study, some participants were randomly assigned to receive extra help either through e-mails or in meetings; they got weekly tips for the first month, followed by monthly updates after that. A control group simply received a quarterly newsletter with eating and exercise tips.

Participants were instructed to keep their weight within three pounds of their current weight. That range was dubbed a healthy green zone, like the signal on a traffic light. Those who stayed in the zone received congratulatory phone messages and small rewards, including green tea, mint gum and even a dollar bill.

Adding three to four pounds moved participants to a cautionary yellow zone, where they were advised to tweak their eating and exercise habits to return to their starting weight.

Regaining five or more pounds landed participants in the red zone. They were told to start dieting again and were urged to open an emergency toolbox that included a pedometer, a diary to record their food intake and a meal-replacement shake. For inspiration, they received a copy of their own weight-loss success story that they had written for researchers as the study began. They also were offered one-on-one weight-loss counseling to get back on track.

Those strategies enabled Ed Messier, 64, of North Smithfield, R.I., to maintain his 56-pound weight loss during the study. Messier and more than half of his group -- those who met regularly throughout the study -- succeeded at stabilizing their weight. So did nearly half the e-mail group. (By comparison, just about a quarter of the control group maintained their weight, according to the study.)

For Messier, the regular meetings were helpful, but stepping on the bathroom scale daily "was the single most important tool," he says. "I still weigh myself religiously, and if I am up a pound or two and see things going in the wrong direction, I am much more diligent [about eating and exercise] in the next couple of days to make sure that I am not going too far off."

The scale is also key for Susan Kertzer, 57, of Providence, R.I., who carried the scale with her everywhere. "It was comical," she says. (And sometimes difficult, such as when she had to explain to airport security in Paris why there was a bathroom scale in her carry-on luggage.) But by stepping on the scale daily -- and adjusting food and exercise accordingly -- Kertzer not only maintained her weight, she lost an additional 14 pounds during the study.

"Using the scale is very objective behavior," notes the study's lead author, Rena Wing. "It's a lot easier to get on the scale each day than to start writing down your calories."

Wing likens using the scale to reading a thermostat: If the numbers stay steady, no need to make any changes. "But if things are not going well," she says, "then you can go back to writing down the calories" and find ways to boost metabolism with more exercise.

Susan Yanovski, an obesity expert at the National Institutes of Health, the study's funder, says the results prove for the first time "that it's better to catch the little weight regains" before they become big ones.

"The most successful people at weight maintenance are the ones who can stay in that green zone," Yanovski says. "They weighed themselves every day." And, she notes, "they have a game plan for getting back on track." ?