Do the twist:** If your subject is standing, have them take a step back with either foot. If seated, have them sit at an angle. This forces them to twist to face the camera – stretching their torso and hiding bulges.
Get high:** Shoot from a little above your subject's eye level. This elongates the face and makes them face up just a tad – helping conceal a double chin.
Narrow down:** Short or narrow lighting involves illuminating the side of the face that is turned away from the camera. This puts a large part of the face in shadow, making it look quite a bit thinner. You can use a window, a lamp, or an off-camera flash (like the Vivitar 285HV Flash) to do this.