Send out your thank you letters no later than 24 hours after your interview has ended.
Always keep your audience in mind. Address the issues and the concerns that were mentioned during the interview.
Use the thank you letter as a follow-up "sales" letter, in which you restate your reasons for wanting the position and remind the interviewer why you're qualified.
Mention anything of importance that your interviewer might have neglected to ask in person.
If you're only writing a few sentences, send a handwritten note. Otherwise, send a typed, formatted letter (see sample letters below).
Thank everyone who contributed to your job search.
If you decide after the interview that you don't want the job after all, write a respectful note withdrawing your application.
Choose your words carefully when using e-mail. E-mail creates an instant written record, so don't let the speed and the ease of sending it blind you to the fact that you will be judged on what you've said and how you've said it.