Nine guidelines for writing effective email messages

"Keep it brief: I never make it through a long email. I find myself scanning, and I miss important details. You’re not writing a book or a love letter, you’re sharing information. Share the information and move on. If you write more than two or three paragraphs, a face-to-face meeting or conference call might be better.

Include a succinct subject: Long subject lines are as bad as no subject at all. Pinpoint a few keywords that convey the email’s purpose. [...]"

http://www.techrepublic.com/blog/msoffice/nine-guidelines-for-writing-effective-email-messages/5154