"In my research, 9 out of 10 people who've lost weight and maintained it exercise regularly and make it a critical part of their lives," says Fletcher. Even if you've never been a fitness fanatic, it's not too late to get moving. Even taking a few 15-minute walks throughout the day will be a move in the right direction. Once you start to enjoy the mood-boosting and calorie-burning advantages of exercise, start looking for ways to keep your workouts interesting. Join a local gym and try a variety of classes and cardio machines, find friends to walk with, or experiment with at-home exercise DVDs.

They don't solve problems with food

Almost everyone is guilty of occasionally drowning their sorrows in a pint of Ben & Jerry's or taking out frustration on a batch of brownies, but thin people definitely don't make it a habit. "They tend not to eat purely for emotional reasons," says Fletcher. When you are upset (or bored, lonely, frustrated or angry), she suggests asking yourself: Is food really going to solve the problem, or will it just end up making you feel worse after you finish eating? Chances are, the food won't fix what's bothering you, so it's important to come up with a list of other small pleasures you can turn to instead of food. Some ideas include going for a walk, watching a movie, calling a friend, playing with your kids, or taking a bubble bath.

They stop eating when they're full

Most thin people are not members of the clean-plate club. Instead of mindlessly eating however much they are served, they pay attention to internal hunger cues and satiety. "Thin people are tuned into noticing when they are satisfied, and they stop eating even if there is food left," says Jill Fleming, M.S., R.D., author of Thin People Don't Clean Their Plates (Inspiration Presentations Press, 2005). "They often report that they don't like that feeling of being uncomfortably full, so they've learned how to stop before they reach it."

They eat breakfast

According to the National Weight Control Registry, which tracks the habits and strategies of more than 5,000 people who have maintained a significant weight loss, nearly 80 percent of these successful losers eat breakfast every single day. And most of their naturally lean counterparts do the same, and make sure that eat within about an hour of waking up. "Breakfast is literally breaking the fast of the night," says Fleming. "Until you send food into your system, your metabolism doesn't really start to kick in."