The most important thing is not to preach. It's also about not critisizing or making your loved one feel bad about themselves or become defensive. I would suggest inviting them to do something active with you, such as going on a walk-tell them you have a lot going on in your life and you just want to relieve some of the tension while talking it out with them. You could also make plans to play tennis, go shoot some hoops, even just going to the mall and browsing through the stores, while going from one end of the mall to the next. It's simple, really. Talk a little about how working out in any way releases feel-good endorphins that relieve stress, give you more energy and a more positive outlook on life. But never, never tell them that the way they're living is gross, irresponsible, unhealthy, or destructive. This is about a positive change. Remember, you attract more bees with honey then vinegar.