The best way to let a prospective employer know you appreciate being granted an interview AND are very interested in employment is to follow up with a thank you letter.
Although simple to do, a thank you letter is very important in the job-search process. A thank-you letter after your interview serves two purposes.
1) A thank-you letter shows that you respect the interviewing process, as well as respect the authority of the interviewer. The thank-you letter tells the interviewer that you aren't some cocky individual who just assumes you got the job, but, rather, that you realize there are others competing for the position and that you will go the extra step to set yourself apart from the competition.
2) A thank-you letter also brings you back into the interviewer's focus. Studies show that people tend to remember the first and the last of something, whether it be a speaker, a list of choices, or, in an interviewer's case, the first or last job candidate. It's called the law of primacy and recency, where the first and the last get remembered more. So, if you're a job candidate who interviewed somewhere in the middle of a string of interviews, you may not stand out as much. Of course, you hope your resume will still help you stand out, and that the interviewer reviews all candidates equally. But, again, you personally may not stand out as much if your interview was stuck somewhere in the middle. A thank-you letter will help bring you back into the interviewer's mind.