Recently study shows that only about 4% of job applicants send a thank you note after the interview. Speaking of an opportunity to gain a huge competitive advantage at nearly no cost at all!
Plan to send out your thank you letter as soon as possible after your interview, preferably within 24 hours.
If it is possible at all, send your thank you note by email. The body of the letter should include why you are a well qualified solid fit for the desired position and enthusiastically thank the interviewer for their time and consideration.
Spell check and proof read your thank you letter! Then ask someone else to proof read it for you. That way you will be sure it's perfect.
In conclusion, it is always appropriate to say thank you after your interview. It is not only a polite thing to do. It may elevate you right to the top!