So, I was thinking about writing some thank you notes to a few professors. One really is helping me, and the other two have made a huge impact on my life. How should I compose them?

Thing is, I would love to stay in touch with these people who have basically encouraged me and made me very proud of my work. They're fantastic teachers. How should I do this without acting creepy? Should I even do this at all?

Professors/educators/anyone, feel free to chime in!