Showing less of the upper body and shoulders can help get good results. Moving in closer for a headshot, too. Take more shots with animated or varied expressions and edit rigorously on the back of the camera. See what looks best, then do more like those. Angle, positioning, and lighting can also help. Standing with good posture, not sitting, can work. To de-emphasize the neck and under the chin, a helpful technique is face the camera and slightly project just your head towards the camera. Resembles a turtle sticking its head out of its shell. After the shoot, Photoshop can do slimming and slenderizing as well.