I like this useful post from Chris Blattman though it is a bit fussier than necessary.
#1 is very important - I received an email from an undergraduate at Western Ontario back in the day whose email address involved the term "longdong". I think probably that account is best used for other things than emailing your professor.
The other most important ones have to do with wasting people's time. Don't do it!
I am fine with emoticons in moderation and with quotes from either the famous (or the infamous - those are often better). My favorite, from a long time ago, was "It doesn't take all kinds, but we have all kinds."
I also think some of this varies with how well you know your professor. I would treat Blattman's comments as a guide for initial emails, not emails with a faculty member that you know reasonably well.
Via Sue Dynarski
Who was my favorite student this term?
11 months ago