Photo: Jeff Harris Studio

1. Set an example: "Kids model their parents—they learn healthy, as well as unhealthy, behaviors from them," says State University of New York at Buffalo psychologist Leonard Epstein, PhD, a pioneer in family-based obesity prevention. Telling a child to go out and get some exercise is far less effective than asking him or her to join you for a bike ride. 

2. Set goals: Consider having everyone in the family sign a healthy-living contract. In the short term, don't focus on weight loss. Instead, have everyone agree to specific daily goals, such as recording a certain number of steps on a pedometer. 

3. Use nonedible weekly rewards, says Daniel S. Kirschenbaum, PhD, clinical director of Wellspring weight loss programs. A small gift or a few dollars—or even allowing a child to have a friend sleep over—will help reinforce the importance of working toward specific goals. 

4. Sit down for a family dinner: Kids who do this at least five or six nights a week are a third less likely to be overweight than children who never eat dinner with their families, according to a 2006 University of Alabama study. Other research shows that kids who eat more family meals consume less fried food and artery-clogging trans fat and saturated fat. Get started with quick, easy recipes for the whole family.

See how one family lost the weight—and kept it off

Timothy Gower is a journalist and the author of several books on health.