I used to have a printout of this on the office divider at my desk: Link

1. Read the Roman Raphaelson book on writing. Read it three times.
2. Write the way you talk. Naturally.
3. Use short words, short sentences and short paragraphs.
4. Never use jargon words like reconceptualize, demassification, attitudinally, judgmentally. They are hallmarks of a pretensious ass.
5. Never write more than two pages on any subject.
6. Check your quotations.
7. Never send a letter or memo on the day you write it. Read it aloud the next morning - then edit it.
8. If it is something important, get a colleague to improve it.
9. Before you send your letter or your memo, make sure it is crystal clear what you want the recipient to do.
10. If you want ACTION, don't write. Go and tell the guy what you want.

In my experience, workplace email culture often fails dismally on points: 2, 4, 7, 8, 9 and 10.