Here is an interesting blog post by Michael Leddy on the “proper” way to email professors (or anyone else if you want to sound professional in my opinion).
Orange Crate Art: How to e-mail a professor:
I would greatly advice you to check it out when you have a few minutes to spare. Remember you are not sending me a text message, a 140-character tweet, or a telegraph. I will delete “blank” or “improper” emails with just attachments. This semester, I have almost 100 students so I would greatly appreciate some detail when you email me.