An interview thank you note is only effective if it arrives in time to have an impact on the employer's hiring decision. For that reason, the employer must receive your letter within 24 hours (i.e. the next business day) after your interview.
There are two effective ways to submit your letter and one strategy you should never use.
Two effective ways to send your thank you letter:
1. Drop it off in person
This is a good way to submit a thank you note because you'll get a chance to visit the place of business one more time. If you live within a reasonable distance from the place of business, and if it seems appropriate to come back the following day to drop off your thank you letter, this can be a good strategy. Be sure to put your letter in an envelope and clearly address it to the interviewer. You will likely end up giving the letter to a receptionist, so be polite and be sure to let the receptionist know what it is so he or she will be able to pass it along in a timely manner.
2. Send an interview thank you note via email
When you get a copy of the interviewer's business card, his or her email address will very likely be on the card. It is perfectly acceptable to compose a gracious thank you email and send that to the interviewer. Do not send the thank you letter as an attachment in an email, instead, write your thank you directly in the email. People often avoid opening attachments that they are not expecting, so your thank you may not get read if you send it as an attachment.
Never send interview thank you notes by regular (snail) mail
Clients are often surprised when I tell them this. They often feel that faxing a thank you note or sending an email is too informal. However, a sincere fax, email or hand delivered thank you note is perfectly acceptable at this stage in the hiring process, and assuming you send it early the next business day, a fax, email or hand delivered letter will get there in time to have an impact on the employer's hiring decisions.
If you send a thank you letter by regular mail, it will take several days to arrive at the place of business. By that time the employer may have made a hiring decision, and it will be too late for your interview thank you letter to have any impact on the employer's decision.
So, hand deliver, email or fax a sincere, gracious interview thank you note within 24 hours of your job interview, and you will make a positive impression on the employer.