- Strunk & White. Write crisply, punctuate, and capitalize. Don't make mistakes.
- Cover one topic per e-mail. If you have lots to say, break it up.
- Use the subject line to articulate your main idea, question, or action item. Your recipient should know exactly what they're getting before they read the message body. It might help to write this last.
- Favor lists. Lists are easier to write than paragraphs, and easier to parse. They also encourage inline replies. If you have to use paragraphs, make them absurdly short: one or two sentences each.
- Reply inline if possible. It's faster to address each of your correspondent's points in turn than it is to compose a coherent reply to all of them; inline replies are also easier to read, because they're right beside their referents.
- Paste whatever you can. Attachments are clumsy—they must be virus-scanned, downloaded, and opened (and Microsoft Word takes a while to launch).
- Respond within 24 hours, if only to say "I'll get to this later." But be sure to give a conservative time frame for your more thorough reply.
[If you're writing a long, letterish e-mail, then to hell with it all (except #1, of course)—these are so rare that no one will begrudge you if you write with abandon.]