A nice clean, beautiful face shot is important. I recently shot one and to relax I laid on the couch and put my face against the arm of the couch and focused on the lens of the camera and looked through it as if I was looking ito the eyes of someone else and telling them " I understand you." It might sound corny but if you think of a calm situation when you are shooting the images WILL look better, calmer, and more like an ad. Also bringing the tear sheets with you from magazines that show face shots of models for creams and makeup is a good idea.
Relaxing and giving a calm look is work! It does take a minute to get into Model-mode and be able to give your honest expression for a really nice headshot.