TL;DR: raise her self esteem. Make her think weight loss is her idea.
Long version: Former manipulator here, with a track record of getting a few girlfriends to loose major amounts of weight. I always believed formerly overweight women had more hobbies and interests than women that had always considered themselves attractive, and dated often. These tactics will be shady to some, and involve the long con. This worked for more than a few women I dated but of course won't work on everyone.
Tell her often that she looks pretty. Seriously. When a woman starts feeling good about herself, then she'll place more importance on her looks. It's positive enforcement. Compliment her often when she does her hair, make up, new shoes, whatever. If she NEVER does these things, invite her to some place fancy, and dress yourself nicely. She's more likely to dress up. When she does, compliment away.
Make her feel good about herself (Um, you should be doing this anyway, btw.) After that, depending on her personality type of course, she'll actively start trying to look her best more often.
Only after she brings up weight loss will you suggest a professional means. A majority of women I've come across are self conscious about their bodies already, so it won't take long. Seriously you might have to wait two weeks maximum. When she mentions her weight, tell her that she looks beautiful the way she is, BUT if SHE thinks she wants to lose weight then you'll support her decision. BUT you don't want her to do it in an unhealthy way. Weight Watchers is cheap, and works really well for people new to the weight loss word. I've been around women that have tried other systems; weight watchers is the most realistic. Let them give her guidelines.
Never hover and ask if she is "supposed to be eating that." The mandates are coming from someone ELSE, remember that. YOU don't personally care.
Learn the stuff with her, and when you're out, order your food as if you were on the diet also. Someone in her circle has to be doing this also, to reinforce the habits. My appetite shrunk from ordering kids meals with an ex, it happened in less than two weeks.
Reward system: With one ex that was 80lbs overweight, I told her that if she hit her first target weight of "only" 20lbs, I'd let her pick out what ever make up she wanted, and I'd pay for it. She hit the goal in maybe 2 1/2 months. Another ex lost 20 in 3 months also, but that was just from eating less and hula hooping. Seriously. Fads work, but try to steer them towards something that is long term. Hula hooping for hours on end is not a long term solution, by the way.
People have to be rewarded in something that's relevant to them. Also, whether it's losing weight, or losing debt, it has to be tackled in small chunks. 20lbs is realistic. Losing 80lbs "isn't." But of course a person can lose 20lbs four times.
Anyway, there's more to it, but expand upon what I wrote and you'll be headed in the correct direction.
After hearing so much crap about my system from one female friend that I respect, I stopped dating overweight women entirely. I don't think that's a win for everyone, as some women just need a little nudge to start what they were going to do anyway. But if you're already in a relationship with a woman that otherwise matches you, try these methods out.