Follow Up Letters (aka Thank You Letters)  are key to a successful interview. I found an article reviewing the guidelines of writing a followup letter from About.com.

Since less than 10% of interviewees ever follow-up with a thank you letter, doing so can help you stand out from the crowd. Here are some tips for writing your thank you letters, as well as links to examples of thank you letters to use as a guide.

How To Write A Thank You Letter

You should plan to send a thank you letter within 24 hours of your interview. While some professions would expect a mailed hard copy, in the technology industry, emailed thank you letters are considered the norm.

When you write your thank you letters, use these guidelines while writing:

• Express Your Enthusiasm: Convey your interest in and enthusiasm for the company and the position for which you interviewed. Try to be specific about why you are interested and how you are a good fit for the team.
• Address Unresolved Points: Address any issues or questions that came up during the interview that you feel you did not fully answer. This letter is your last chance to make a positive impression on the interviewer.
• Personalize It:You will likely be one of many interviewees, so you need to set yourself apart from the other candidates so they will remember you when you leave. In your letter, highlight a key point from your interview that you believe the interviewer will remember, and therefore remember you. Additionally, if you meet with more than one person, consider sending them all thank you letters, each one a bit different; you may not know exactly who in the group will be making the decisions. Getting a business card from each interviewer will help you with names and titles when you sit down to write your thank yous.
• Reiterate Your Expertise: If the company communicated its specific needs, issues or challenges, use your thank-you letter to demonstrate how you can meet those needs.
• Highlight Your Successes: Similarly, if the company communicated its ideal qualifications for a candidate, use your thank-you letter to outline how you meet or exceed those qualifications.
• Proofread, and then Proofread again: Make sure your thank you letter conveys a professional image by ensuring it is free of typos and grammatical errors.

cited: http://jobsearchtech.about.com/od/resumesandletters/a/thankyous.htm

Be genuine when composing a follow up letter, especially when dealing with recruiters. Some recruiters may get several follow up letters from many other candidates so you want to stand out.

Sometimes you can use follow up letters to clarify things in your interview. I had an interview that went bad because I was so nervous. But I used the follow up letter to make up for my mistakes. I mentioned that had I not been so nervous you may have seen how much of a hard worker I am. The good news is it worked, and I got another interview that went 100% better. Being genuine is important.

Good Luck!