I'm a teacher at a small Swedish university, and I often communicate via email. In the beginning of a course, I tend to try to be somewhat formal, which isn't really my personal style, but soon I drop all the "Dear" for "Hi" instead. In class, I'm informal and quite personal.
My "problem" is the sign-offs. I've simply more or less had enough of "Regards" (although I use it anyway to be safe) but don't really feel too comfortable with BR/Best/Cheers either, at least not in the beginning of a course. (My students are all second-language learners of English.)
Any better ideas, anyone?