Controlling the angle of the primary light source illuminating your face can go a long way to making your face look a bit slimmer. The last thing you want is for on-camera flash to be the primary light source. Rather, find a bright light source that allows you to stand next to it at about a forty-five degree angle to your right or left, with the light just a tad higher than your face. The resulting shadows in the image will tend to minimize the edges of your face and make it look slimmer.
Another thing you can do is extend your neck forward and raise your chin when looking directly towards the camera. Although it feels unnatural, it looks better because the skin under your chin is stretched tight instead of hanging more loosely.