With the right pose, anybody can look like a model that has done photo shoots for years. Standing with both hands on your hips is one of those poses which make the subject look casual and comfortable in front of the camera. This is a pose which especially looks good on slightly overweight women, as it breaks down the torso from the arms, giving an illusion of slenderness.
To pull off such a pose, have the subject stand with her weight on one foot, with the other leg slightly bent at the knee. And then have her put both of her hands on her hips. For a somewhat dramatic effect, you can have your model look away from the camera.
If you want to further enhance the illusion of weight loss, take the photo while standing on a stool, so you are higher than floor level. At this angle, the subject will appear as if she has shed a few extra pounds.