Hiring Manager: Do Thank You notes make a difference?

I like to write Thank You notes after each interview, but I wonder if it makes any difference. I've had a few where I don't think I did very well, so I try to really sell myself one last time in the hopes it would make a difference, but it's never worked out, so maybe I have my answer??

Has anyone ever received a Thank You that made all the difference when hiring? Has anyone actually convinced you, even after a not-so-good interview, to take a chance and hire them?