Most people dont want to look fatter than they really are. So get your subject to turn their whole body three-quartes to the camera , facing roughly in the direction of the main light , if possible. Ask them to turn their faces back towards the camera. This makes the body and face look narrower and can even work wonders for beer bellies! A little tilt of the head can really help the pose , too
Then there is problem of what to do with arms and hands. Hands on hips or thumbs tucked into pockets can look good. Arms folded can look really confrontational which of course may be the effect you're after. A better option to relax your subhect can be to get them to lean on something or even perch on the edge of a seat to create interesting angles in the composition.
The height you take the photo from can also affect the feel of the picture. Shooting from eye level gives great eye contact. Shooting from slightly above , looking down to your model can make them look passive and is agood way to disguise double chins. However shooting from slightly below can make the model look more dominant or powerfull.