Merlin Mann and I have been talking for a long time about what makes an easily answered email, and he's written up a fantastic primer on writing good emails, especially emails to colleagues and strangers. This is the kind of thing I want to see printed as a pamphlet and handed out on busy street-corners: if ten percent of it were taken to heart by ten percent of the email-sending world, humanity would be immeasurably benefitted.
You can make it even easier for your recipient to immediately understand why you've sent them an email and to quickly determine what kind of response or action it requires. Compose a great "Subject:" line that hits the high points or summarizes the thrust of the message. Avoid "Hi," "One more thing...," or "FYI," in favor of typing a short summary of the most important points in the message:Link
* Lunch resched to Friday @ 1pm
* Reminder: Monday is "St. Bono's Day"--no classes
* REQ: Resend Larry Tate zip file?
* HELP: Can you defrag my C drive?
* Thanks for the new liver--works great!
In fact, if you're relating just a single fact or asking one question in your email, consider using just the subject line to relate your message. As I've mentioned before, in some organizations, such emails are identified by adding (EOM)--for end of message--at the end of the Subject line. This lets recipients see that the whole message is right there in the subject without clicking to the view the (non-existent) body. This is highly appreciated by people who receive a large volume of mail, since it lets them do a quick triage on your message without needing to conduct a full examination.
Sadly, good email subjects have become something of a lost art, especially among more recent additions to the Interweb. It's a pity, because you're far more likely to get a favorable response from a busy person when they can quickly grok your message.