The Guardian:

Perfect, your cover letter and resume secured an interview — the interview went brilliantly. You’re confident they are interested in you. Now, you just have to wait for the offer, right? WRONG! Don’t stop the campaign yet. It is a known fact that many hiring decisions rest on which candidate sends a thank you letter. Put the situation in perspective. Imagine a hiring manager, undecided between two “equally” qualified candidates. Then, one candidate sends a well-written thank you note, and the other doesn’t. Who do you think will receive the job offer?

Aside from the fact that a thank you letter is a marketing tool, it is common courtesy for job seekers to send a letter to all interviewers. Hiring managers know that those who send a letter are informed and following standard protocol. Timeliness is also important. It is best to write your letter and send it within 48 hours of your interview. So how do you create an effective, unique thank you letter that will show you remain interested in the position? In an effective thank you letter, there are three or four paragraphs. The best advice is to make sure the letter is unique, not generic. Make sure you address the letter to the person(s) who interviewed you. Include the interviewer’s name, title, organisation, and complete mailing address. Ensure correct spelling of all names — if you are unsure of spelling, you may be able to obtain this information from the company secretary.

Opening paragraph: Express your appreciation for the interviewer’s time and the opportunity to meet. Second paragraph: Reinforce your understanding of the job’s requirements and emphasise your qualifications. Include any information you forgot to mention in the interview, if necessary (but be brief). Maybe include additional information about any research you did after the interview. Express appreciation for any opportunities presented during the interview, such as touring the facilities, meeting other employees, or reviewing specific company situations. Re-emphasise your most important skills and qualifications and how you expect to contribute to the organisation.

Third paragraph: Only use an additional paragraph if you need to correct any significant misunderstanding that may have occurred during the interview. Alternatively, use this paragraph to counter any objection the interviewer raised about an aspect of your background. Be extremely careful wording this information. Final paragraph: Express continued interest in the position and the company.