A typical marketing portrait is cropped to head and shoulders. In the studio, this will be on a plain background, white being the most requested. In the work place, distracting backgrounds are usually blurred out.
The photographer will normally ask you to turn your body 45 degrees to the camera. You head should then turn towards the camera. Leaning slightly forward from the waist will cause the neck to elongate creating a strong and confident image. This will also help to reduce double chins and neck wrinkles (see next tip).
Look straight into the camera, making good eye contact - squinting ever so slightly may improve the look of the shot. When having your photos taken, imagine that your best customer has just walked through the door - the resulting headshot should portray a friendly, approachable, smiling individual.