My clients often ask me about thank you note etiquette, i.e., what is de rigueur?
So, here are my Three (3) Ps for Thank You Notes:
(1) Practicality. All other things equal, a Handwritten Note > E-mailed Note > No Note. I've encountered few decision-makers who feel that thank you notes are a waste of time, but they are the exception. My preference is to send a handwritten note, which demonstrates more thoughtfulness, gratitude, and genuine interest, than an e-mail.
(2) Personality. Reference a specific interaction where the firm went out of its way for you during the interview (e.g., thank you for taking the time to show me around the office). Then, remind the interviewer about a specific skill that left an impression on him/her (e.g., I appreciate that you feel my design skills are of value to your operation). Also, reference one of the interviewer's personal interests that you touched upon during the interview (e.g., the fiction novel I mentioned about the unnamed robbers who targeted Bob Marley is called: A Brief History of Seven Killings, and includes a ton of patois). On an aside, the book is magnificent.
(3) Punctuality. Regardless of whether you send a note via snail-mail or e-mail, do so within 24 hours.