I've spent a lot of time writing email the past few weeks, so this is on my mind.
1) Keep your work email separate from personal. People change jobs, your friends shouldn't lose track of you because of this.
2) Don't use the email account that comes with your ISP subscription. The next time you move or change ISP you will be forced to get a new email account. Combined with (1), this leaves two options: pay for a hosted email solution, maybe with a personal domain-name, or use one of the free email providers. I suggest Google's gmail.com, which offers IMAP connection so that you can check you mail using your favorite email client (Mozilla Thunderbird, Apple's Mail.app, Novell Evolution, Microsoft Outlook, Novell GroupWise, et cetera) as well as on the web.
3) Configure your email client to send plain-text email by default. This usually produces much smaller message sizes and can easily be read by the most email clients.
4) When replying to a email, delete most of the quoted message, leaving only enough to give your reply context. At the very least, delete the signature.
5) If you are not replying to something in a previous message, don't hit reply; instead, compose a new message. This makes a new thread in clients that organize mails into threads.
6) Don't top post.
7) If you are going to compose your message in a word processor before sending it, copy-and-paste it into the email's body instead of sending an attachment.
8) If you must send an attachment, use an open file format.
9) This is one I am very guilty of. If you don't have time to compose a proper reply to an email that requires a reply, you should shoot off a quick acknowledgement message.
10) If you are at all technically savvy, go ahead and install GnuPG (if your system didn't already have it) and configure your email client to make use of it to sign your emails.