In the current issue of People...The Essential Guide to Email for Office and Home, David Shipley and Will Schwalbe write:

1) Do not say please
Instead of seeming polite, adding 'please' can come off as a sign of frustration. It is difficult in writing without seeming obnoxious. And saying "Thank-You" in advance while asking for something is off-putting: " it's a command crudely cloaked in premature gratitude."

2)Forget the Flag
Flags and priority arrows are annoying.Is your e-mail really more important than anyone else's? Convey urgency in the subject line or within the text of your e-mail.

3) Put your boss first
You may not realize it but people actually do pay attention to whose name comes first in the TO field. Order recipients by rank.
4) Speaking of the boss...she might be reading
According to a recent study, 38% of companies employ people to monitor their employees' e-mails. Even personal e-mails are fair game if they are sent from company computers.
5) WHEN YOU ARE MAD, PICK UP THE PHONE

Well....I am guilty as charged with #1, and on occasion, of NOT doing #5....