Make sure when you shoot, you are in control. I don't just mean bossing the photographer around and speaking up about the shots you want and need. I mean...when you shoot, shoot to please you, not just the man behind the camera.
Shoot to get a photo that will tell a story and focus on your eyes, your expression, the face, deep breathes, and relax. It is hard sometimes to relax and give a serious face, sometimes it seems serious means stiff, but if you exhale, close your eyes and open slowly...and stare through the lens, like your eyes are speaking you will give a powerful shot.
Alot of times the Internet model entrance leaves girls thinking it is all about pleasing the photographer when really it is about please you, feeling enpowered!