A quick run down of what I think is most useful and important:
Write in grammatically correct English. I am not fluent in text-speak. I ignore occasional typos, I don't mind the occasional emoticon ;), and I don't want spend a lot of time or energy scrutinizing your emails, but the minimum requirement is that you communicate effectively. Moreover, your emails reflect your intelligence and your professionalism. Get in the habit of writing grammatically correct complete sentences now because it will help you in your career.
Be respectful. When you write me or your teaching assistants, you are usually asking for something. It is therefore in your best interest to be respectful. Some advice from my mother: If you can't say anything nice, don't put it in writing. Being respectful also includes using salutations and appropriate titles.
Dear Dr. Nattrass -
I was wondering if you will be posting homework 7 soon. I would like to work ahead over the break.
Most of the people teaching your classes have doctorates. The title "Dr." is therefore a good default. You will very seldom offend someone who does not have a doctorate by addressing them as "Dr." "Professor" is another reasonable alternative. You should only address your college instructors as "Mr.", "Miss", "Ms.", or "Mrs." or by their first names if they have introduced themselves as such. Do not use any of these titles ("Mr.", "Miss", "Ms.", or "Mrs.") for me - I find all of them disrespectful, each for different reasons. Generally you can address teaching assistants using their first names. If you aren't sure how you should address one of your instructors, ask.
It is also generally a good idea to include a salutation ("Dear") and end with something like “thanks”. I'm OK with emails that start with “hey” or “hi”, but these are less formal and some professors find them too informal. “Thanks” or “thank you” is usually appropriate because you are usually asking for something. “Sincerely” sounds a little too formal for an email to a professor for my taste. “Cheers” and “Greetings” are also common.
Please do not send me “thank you” emails for routine things like answering questions about the class. If you write me a separate thank you email, I assume that this new email is more work that I have to do and it takes me time to sort it to figure out if I need to respond to it.
Be concise and only ask questions you can't answer yourself. It is not uncommon for me to get 30-40 emails/day about class and particularly in the first couple weeks, I may write up to 50 individual replies. It is rare that any subject warrants an email longer than a short paragraph, particularly because all questions about the material should be directed to the discussion forum. Many questions are already answered on the syllabus. Of course the people who didn't read the syllabus probably aren't reading this either. Also the more time I spend answering questions that didn't need to be asked, the less time I spend answering substantive questions about physics. I do not need detailed information on your medical condition if you need to be absent from class for medical reasons.
Emails are forever. Anything you put in email can be forwarded to someone else. While most communications between students and professors are protected by FERPA and therefore not available to the public, I can and frequently do forward emails from students to teaching assistants and sometimes other faculty in order to coordinate the class. In the case of behavior that violates the student code of conduct, emails may provide evidence of such behavior. Do not write emails in anger. Even though student/professor communications have some protection under FERPA, most email provided by employers is not private and employers frequently monitor employee email. It is therefore a good idea to get into good habits now.