<p>Without a doubt, the finest way to thank a professor is to write to them, providing details about the things you learned in their classes, and the new ideas you’ve developed with their guidance.</p>
<p>Such a letter (or even an email) can be included in their dossiers when they are up for promotion (or for younger professors, tenure).</p>
<p>So don’t just thank them for the letter of rec, thank them for their contribution to your education.</p>