I find that limiting the number of hours allowed in front of the TV makes a big impact. It's also important for parents to set the right example in making their kids lead active lives. Before or after dinner, go out for a 15 minute walk instead of turning on the TV.
This infographic shows how the design of communities can help families be physically active. The presence of parks, trails, playgrounds, sidewalks, and bike lanes encourage walking, biking, and play among both kids and adults. And when communities are safe, well-maintained and have appealing scenery, children and families are more likely to be active.