Quite often the secret to a successful portrait session is establishing an element of control in the session and demonstrating an appropriate level of confidence in your shooting abilities. If a photographer doesn't establish a degree of self-confidence in his or her photo abilities, the photo subjects will have a difficult time relaxing for the shots. Even if things don't go exactly as planned for whatever reason (uncooperative weather, equipment failures, etc.), it's important to show your portrait subjects that you have control over the session and that they should trust your abilities as a photographer to capture great images. Sometimes it's just a matter of showing that you know what you are doing -- demonstrating an eye for detail, posing the subjects in a flattering way, etc. -- and you'll gain the trust of your subjects. When they trust you in your abilities...they'll feel more comfortable and relaxed having their pictures taken.