If you have made it all the way through the resume gauntlet and come out on the other side of an interview make sure to say thank you. The best way to do this is with a tradition post-interview thank you letter. Doing this is not only a case of good manners, it also allows you to subtly keep your name in front of the hiring committee and highlight anything that went particularly well in the interview.
Before you send that letter off be sure to follow these five general guidelines to be sure you get the most out of your short missive.
Keep it short. A post-interview letter is not the same as a cover letter. You do not need to go over all of your accomplishments and credentials again. Keep it brief and to the point. Tell them you appreciate the opportunity to meet with them, and that you are available to answer any follow up questions. I suggest something along the lines of 2-3 paragraphs at most.
Be professional. While you might be tempted to write something very casual to come across as personable, remember this is a professional thank you letter. Use a formal letter template with the traditional header and salutation. Traditional fonts like Times New Roman or Arial, size 10 or 12 are best for this type of writing.
Send it soon. This type of correspondence needs to be timely if it is going to have the desired impact. You want them to receive the letter while your interview is still fairly fresh in mind. I recommend writing your letter the day after the interview and them mailing that day it. This way they will be reading it in about 3 days. This is enough time for them to still remember the interview, but not so soon that it feels like overkill.
If possible, personalize. While you don't want to sound unprofessional, after you have met and spoke with the interviewer you can assume a level of connection that wasn't there in the cover letter. So if there is a quick way to personalize the message go ahead, but do not force anything. Perhaps someone mentioned there was a company softball team. This would be the time to mention you were the starting short stop on your high school baseball team.
Proofread. Then proofread again. This is the oldest advice in the book, but that is only because it is that important. Don't blow it with a typo in your thank you note.
A note on email: If you know the hiring decision is going to be made in the next couple days, then it is appropriate to send a post-interview thank you letter in email. However, all of the above still applies. Be professional.