there are some really good tips here and i agree with most of them. now of course i have to give my own version. first of all, know her, identify her personality and her attitude. i believe it is not for everyone to be overweight or obese and i say this for myself. it is not a matter of how i look i just feel horrible when i am overweight, so now, first get to know your partner, if she has been let's say, an example, all her life, a runner, trim runner, under 130 pounds, with no really big appetite and used to run miles a day, without a craving for fattening meals, you don't really have a chance that she will gain permanently or happily. so you either join that lifestyle or go away.
but if you see hope... in the shape of a big appetite, a tendency to gain weight, a craving for sweets and fast food and not being particularly fond of exercising, looking for shortcuts to be slim because that is in and not because she is slim in her genes then my friend you are on to something. after that, yes, all of the above applies, do not ever talk about her weight, if she talks about dieting let her, encourage some exercise specially if you do exercise. have her favourite snacks available and invite her to some fancy dinners ever now and then. over 'soft' i prefer curves, but somehow let her know how sexy she is to you and yes, give her a sense of total stability in the relationship.
lights on. cellulite on. grab everything you can and point out how hot she is.