Do not use profanity. Be very careful when discussing financial or business affairs. Avoid any mention of your private parts. Do not offer any guarantees, or refer to checks that may or may not be in the mail.
Refrain from describing anything or anybody as "free." Abstain from the exuberant use of punctuation marks. Shun simple salutations like "Hello," and opt instead to craft a detailed, personalized subject line.
Oh, and don't ever use the word opt, particularly in conjunction with the words "in" or "out."

This is the advice from a story at Wired referring to how SPAM filters are sometimes taking out real e-mails along with the junk mail.