2.) To Copy?: Easy thing to do is to copy everyone on an email of "importance", but in many cases this leads to too many opinions and a high level individual having to read an email trail that fills up their otherwise busy inbox and morning. Always ask yourself, "Is this issue important enough to copy these people?"
3.) Don't Argue Electronically: It is way too easy and way to convenient to get sucked into an escalating argument over email. Email lack's of context and tone make it difficult to make a point because you don't know how the receiver is going to read it (in what tone). I'm not a statistician, but I would estimate that about 10000.99% of all problems start with an unnecessary email response and the desire to have the last word.
4.) Sleep On It: Everyone has been the recipient of a bold or harsh email and has felt the need to respond immediately just as harshly. Sleep on it. Email and Facebook lack emotion but an elicit a lot. Be the bigger person and consider how important it is to send a harsh response. Consider that another harsh email will likely follow back to you. No one wants that.
5.) Email Is Not A Conversation: It is always nice to have a written record of things, but is email quite as good as talking to someone? I would argue no. There is rarely a replacement for face to face interaction; even a phone call. I often joke that if the phone was invented after email, people would have forgotten about email immediately and been amazed with the technology... especially if it had been invented by Apple.