The worst duplication sin of all is to store similar copies of the same documentation in version control. The root cause here is typically the desire to have one version for coworkers and another for customers, and it inevitably leads to a maintenance nightmare.

Administrators install and configure applications. I'm not just talking about server administrators or people with administrator permissions on a computer, but anyone whose concerns are setup and maintenance.

you should break your writing into bite-sized pieces, often called topics, none of which overlap in content.

A good rule of thumb is to treat everyone as busy individuals whose time is valuable, but another rule of thumb is to break this rule and be more of a jerk if an essential employee isn't giving you the information you need to do your job.

Tailor your approach to the person, come prepared to meetings, batch your questions into related sets rather than bothering people every twenty minutes with individual questions, and thank everyone for their time.

if things change—and things always change—you only have to update the documentation in one place.

How do I install the product? What are the basic configuration options, if any?

Plenty of lightweight markup languages exist, but only three are really worth discussing: Markdown, reStructuredText, and AsciiDoc.

Learn everything about a subject. Write down exactly what an audience needs to know and no more. Make the content beautiful, discoverable, scannable, and searchable. Consider everything a draft, and iterate relentlessly. Make contribution simple.