The desire to lose weight / get fit has to come from within. No amount of nagging by others is going to get someone to put down the Whopper and get on the treadmill.
But there are things you can do, especially if you yourself want to lose weight / stay slim / get fit. Drag your S.O. out for a run, or to the gym. If you're out together and want to grab lunch, don't let her/him go to McDonald's; go to Subway instead (not that you can't find unhealthy food there, but they are more low-fat / low-carb friendly than other places). Set a good example and try to bring your S.O. along for the ride.
Also, point out that the fitter you are, the more stamina you will have in bed. That will certainly get a man's attention.
