The post-interview thank-you note (in scope, it should be more of a note than a letter) serves at least three purposes. First, it shows courtesy, thus adding to whatever rapport you accomplished in the interview. This is important, because a major question in the interviewer’s mind, entirely apart from your technical skills, is whether you’ll be a nice person to work with. You can reinforce this impression by handwriting the message on attractive note stationery, rather than on a sterile 8 by 11 sheet of paper. If your handwriting is truly atrocious, get someone else to pen the note; or perhaps you can figure out a way to feed the stationery into your printer and then hand-write your signature. Don’t use stationery that’s so flowery or cutesy that it’s inappropriate for a business setting.
The second purpose the note serves is to give you a chance to make comments that might reinforce the interviewer’s impression of your technical skills or that might dispel some negative information about you that came out during the interview. Every job-seeker comes out of an interview thinking, “I wish I had remembered to say x,” or “I wish I had answered that question better.” In the context of a note, you have limited space to correct these omissions or gaffes, so choose your words carefully and don’t overburden the note. The note can’t be an after-the-fact substitute for an inadequate resume or cover letter, although it may refer to what was written there.
The third purpose for the note is to indicate your interest in and enthusiasm for the job. Tone is important here. You have to project confidence in your qualifications and not come across as needy.
One other purpose this note sometimes serves is to mention any relevant information that has come out since the interview. For example, if you received another job offer or the Employee of the Month award, this is something you would want your interviewer to know.
Unless you know that the hiring decision will be made very quickly (and usually it takes longer than the interviewer says it will), you should wait a few days before sending the note, lest you appear to lack confidence in your performance at the interview.
These comments apply equally well no matter what your industry is or what level of job you were interviewed for. The only exception worth mentioning is the graphic arts industry, which would call for special attention to the visual appeal of the note.