Getting a job interview is hard enough, so do something to set yourself apart from the rest of the applicants and send a thank you letter after the interview. In the subject line, state that you are thanking the interviewer for their time and attention. In the introduction, use the opening of "Dear NAME". In the body, thank them for their time and let them know you are available to meet again if needed. "Kindest regards" is a great closing to use as your salutation.