Ideally, it is good to mention something brief about the actual interview, some small point that was addressed, or even something the interviewer mentioned, whether about a business or personal matter.
For example, “I do hope that you had a good flight back to Chicago.”
Or, “I hope your daughter’s soccer tournament turned out well.”
Or, “I was particularly pleased to hear that XYZ Company will be opening its new London offices this Spring.”
Or, “I was very excited to hear about all of the new XYZ products that are being unveiled for the Fall.”
The standard suggestion has been to send a Thank You letter after the interview, which is fine. However, many candidates are now finding that they get an EVEN BETTER RESPONSE with a simple Hallmark-type, plain Thank You card with their brief remarks inside.
Apparently, because a card comes in a non-business-style envelope, the fact that that it looks perhaps like an invite, that it stands out from all of the boring business mail they receive every day, and that it shows a more personal touch than a stuffy letter, all combine to create a better result.”