1) Start little by little replacing sweetened drinks (such as soda pop, juice drinks, energy drinks, even Gatorade) with drinks such as iced tea sweetened with mostly lemon juice and maybe one teaspoon of sugar. It may take you and your teens a reasonable amount of time to adjust, but will be well worth it over time, believe me. Many years ago, I made the switch to drinking mostly coffee with cream only, and iced tea with lemon or lime juice (no sugar added). I wouldn't doubt it has caused me to weigh 15-20 lbs less today than if I hadn't switched.
2) Limit recreational computer and TV time to no more than 2 hours a day. This may be a real struggle at first. You may need to reduce your own time in front of the tube and on the computer first, to set the example! Help them get involved in active activities with friends instead; especially those, like sports, that burn more calories and build muscle mass. Have at least one or two nights a week as active family activity nights: swimming, bowling, tennis. This will also help them become more active and physically fit adults one day.
3) Don't allow them to skip breakfast (or just eat something quick and unhealthy). “eating first thing in the morning gets your metabolism going.” Try combining protein and fiber. The protein helps them not get hungry after only an hour or two. And the fiber makes them feel fuller longer. Even eating just a banana or an apple before leaving for school is better than no breakfast. See Little Changes That Make You Healthier! [i]
4) If there are serious weight issues, especially obesity that could soon cause real health issues, you may need to take them to see their doctor. They can provide insights and facts you may not have considered, such as hormonal or glandular problems. They may suggest specific dietary changes. They will know of other treatment options, if that seems helpful.
5) It may seem obvious, but just a reminder then...that setting a good example yourselves is crucial, so they can't come back at you saying, “but YOU watch tv all night long. Why can't I ?!”
[i]Hall, Macey. (1013). Little Changes That Make You Healthier! Seventeen.com. Retrieved from http://www.seventeen.com/health/advice/g300/teenage-weight-loss-tips/?slide=3.