They say that "the camera adds ten pounds". This is not really true of course, but it can easily look like ten or more! There are a few easy tips to make yourself look better in photographs even when they are not taken by a professional photographer.
The first is that you never want your body turned directly at the camera! Always try to set your shoulders at a 45 degree angle to the camera. This will slim your body and shoulders and set a base for your head. Your head should turn toward the camera. Men should tip their head slightly toward the shoulder that is away from the camera or keep it upright. Women can tilt the head toward either shoulder although the should closer to the camera is considered more "feminine".
When seated, bend straight forward at the waist (not toward the camera). Don't overdo this bend and keep your back stretched straight. Try not lift your shoulders around your neck. Leaning slightly on the elbow away from the camera provides a nice base for the image.
When standing, men should keep their feet square and about shoulder width apart. Women should keep their weight on the leg away from the camera and bend the other slightly, even raising their heel a little. This will slim the legs and waist.
Of course, these are just a few of things that your professional photographer should know automatically, but you can use them any time that someone points a lens in your direction!