After every interview, job lead, or good deed performed by someone to help you in your job search, you should write a thank you letter. We consider a thank you letter mandatory after an interview (if you want the job), but it is also a good idea to keep your network functioning well. When people feel appreciated, they will do more for you.
You can send a thank you note in one (or more) of three ways:
The first paragraph thanks the recipient for their time or effort. The second paragraph can be used to add something you forgot to say in the interview, or to clarify an answer or statement you provided. If you do not have a statement like that to make, the second paragraph should emphasize your interest in the position and why you believe you would be a good match. The ending should also include contact information.
Send your note immediately after the interview if possible. If it is a job you are particularly interested in, and you want to devote the extra time, you can always send an e-mail right after the interview and then follow-up with a handwritten note or business letter, acknowledging that you have already sent an e-mail. The letter could start out something like this:
This "double whammy" approach accomplishes two things: 1) it allows you to say "thanks" a second time, showing you are extra thoughtful, and 2) it creates "top of mind awareness" getting your name in front of the decision maker one more time. Also, while the e-mail is the most efficient and immediate way to say thanks, business people typically get 100s of e-mails a week, yet they likely only get a couple personalized letters.