The great and terrible thing about online marketing is how quickly things change. Here's a post from the VerticalResponse Email Marketing Blog that points out some things that not so long ago were widely held to be universal truths -- but may not be so any more: Mythbusters: Do Old Wives' Email Marketing Tales Hold True?.
- Using words like 'free', 'dear' or 'save' in the subject line will cause your email to be marked as spam.
- Test everything!
- Send email only during the week.
- All unsubscribes are bad.
- Sending too many emails too often will make you to look like spam.
As always with things like this, test before you decide to remove it from your rulebook.