Learn to recognize which of your words, phrases and ideas might be unfamiliar to people. You'd be surprised at how many people don't understand what you think they understand. If you use an acronym on page 1, your reader probably will have for gotten what it means by the time it appears for a second time on page 10. Repeat the full name. This goes for people's names too.
Most people are incredibly busy and want to read one page or less.
Most people are not good readers. People have their own ideas about almost everything. They filter your words and ideas through their preconceptions and agendas. Most people like to read common newspapers. After you write something, read it out loud.