The subject line: Please put one in! And make it useful enough that people can tell at a glance what the message is about. This makes it easier for people who sort their messages by subject, and subject lines such as "re: your message," "please read," or "hi!" don't offer any incentive to open your message right away. A lot of people categorically delete forwarded messages as well, so if you are forwarding something and you want it to be read, delete the "Fw" that your email program will add when you hit reply.
The "to" line: This is probably the first blank in the e-mail message, the line where you type the other person's address. If you are replying to someone else's message, this might already be filled in for you. Before you send the message, double check which addressees are on this line. If you are replying to a message that was sent to more than one person, your e-mail program might have put all of their e-mails in, and the message might not need to go to everyone. "Reply" is one command. "Reply to all" is something different. If you get them confused, you'll wind up cluttering everyone's e-mail inbox with extra messages. Before you send the message, do a quick scan of the "to" and "cc" lines and make sure that only one address is there - unless your message really does need to go to the entire group.
Replying: Many e-mail programs helpfully copy the text of the message you are answering into the message you are sending. This can be wonderful because it allows you to quote the message and reply point by point. But it can be tedious if the message you are answering was very long. It is considered polite to "snip" the message: to cut out all the text except the lines you are directly responding to. Long messages that haven't been snipped are slower to load and slower to read, especially if they finally get to your response and find only a one-word "I agree" for their trouble.
Formatting: Try to make your type as easy to read as possible. Tab, bold, italic and other text formatting options may be available in your email program, but if the other person is using a plain-text reader, they won't be able to see it, or worse, their system may translate your formatting into a slew of meaningless cluttering symbols. Use plain text only, and avoid using capital letters for emphasis: that's considered shouting.
Signatures: The signature allows you to add text automatically to the end of every message: e-mail or phone numbers are often included, but some people try to liven up their e-mails by appending little quotes or jokes as well. This can be a fun way to express yourself, but keep it short: under three lines is considered acceptable. And if you include anything beyond your name and e-mail address, change it from time to time to keep things light.
Attachments are useful. They allow you to send documents, photos or even programs to other people. But because they take longer to download and they can contain viruses, they may be viewed as an annoyance by the recipient. Rule number one then is to be frugal. Send attachments only when necessary. Files are fine, and photos are generally all right if you don't send more than one with each message. But that cute little animated fish that someone on your e-mail list sent you - maybe not.
Message forwards: Urban legends, cute little poems and anecdotes and virus warnings can sometimes seem urgent or amusing, but by the time you send it along, most people have already seen it. Keep in mind that genuine virus dangers are often reported in the mainstream media, so it is very unlikely that your e-mail will truly alert someone who didn't already know. And most urban legends on the net are hoaxes.
Before you forward that story about the little girl with cancer who is trying to set a world record for e-mails, or the sister's cousin's friend who was stuck with an HIV-infected needle at the Santa Claus parade, check an urban legend site such as snopes.com to see if it has already been identified as a hoax. And if a message tells you to "forward this to everyone you know," please don't.