Use bold font sparingly but strategically. Bold people’s names when you’re asking them something. Bold topic titles.
Use bullets to make lists.
As short as humanly possible.
Write sensibly. No buzzwords, no jargon, no cutesy.
Be more polite than you think is necessary.* Always use salutations and closings with ex-U.S. correspondence, but it helps with fellow Americans too. They don’t have to be too formal: I generally go with “Hi” and “Best wishes” or “Many thanks”.
Be specific about exactly what you need and how they can help you it.* “If you could provide me with this file, or tell me who I can contact to find it, I’d really appreciate it.” That eliminates the recipient just telling you they don’t have it.
Think of the worst case. If there’s the slightest possibility that it could be misconstrued or used against you later, don’t send it. Pick up the phone or get up and go.