Instead of trying to make them look more trendy, you really should be showing off their real beauty.
Take a look at the old masters' oil paintings and get some ideas on how women should be displayed in more flattering ways. Lighting and settings make all the difference and if you can artisticaly include a message, more power to ya.
One way that I've found is a great way to pose somewhat slightly larger women is to have them lean back in a chair and shoot them from sort of behind and above. This way, it stretches out their neck and makes them look a bit slimmer. Plus, it can hide a lot of other "body imperfections".
Those females in that link I would hardly call "overweight" in the traditional sense (although clinically they are indeed overweight - as is the majority of the country); and theyre hardly posing. . .just snapshots.
I'd check out Lane Bryant website (as its a plus size women store) for suggestions on modern look posing.