I don't know how to encourage someone to start losing weight, but giving her lots of attention, buying her some hot new outfits, having as much vigorous fun with her as you can in and out of the bedroom, and generally getting her to thoroughly enjoy being thin with a touch of regret at all the years she missed out on all that fun through not taking care of herself seems to do the trick in encouraging her to stick with it.
a fine and enviable madness, this delusion that all questions have answers, and nothing is beyond the reach of a strong left arm.