This is very old, 1982 in fact, but 31 years on it is still solid advice, and Oglvy’s twitter account tweeted it.
While is is about memos, it applies just as well to email. And while point 10 is harder in today’s global businesses, with flexible working and distributed locations, it is still the best option. Technology is great, but remember to talk to people!
Keep it short. Keep it sweet. And be clear!