The most important part of writing non-fiction is that it is indeed non-fiction. Once you decide what you're going to write about, you have to research the topic until you know everything about it. Read similar books, related books and tangential information to form a complete and well-rounded context for your subject matter. Non-fiction should be completely factual and evidence based. Use primary sources whenever possible. If you're writing about a person, don't just read other peoples' writings about him; read his letters, diaries and any available writing. Once you've completed a draft, fact check all information against your notes and research.
Unlike fiction or poetry, most non-fiction makes an argument of some kind, even if it's not overt. Sometimes, non-fiction books make several arguments that dispute or go against previously published or believed information. To write non-fiction that people believe and agree with, present your information in a well-articulated argument. For an effective argument, you make a claim (or claims) and provide reasons or evidence for those claims.
Plan out your non-fiction carefully to decide what information to include and in what order. Brainstorm and make notes, breaking your material into sections or chapters. Organize them in a logical way so that the information will build up and make sense. Some writers don't like to work with outlines, but for non-fiction, an outline helps you include all relevant information without skipping around.
Non-fiction comes in many forms, depending on audience, publication or subject matter. Adopting an appropriate style can make or break your non-fiction. Know your audience and cater to them with the way you explain things and the words you use. If you're writing a technical book for physicists, you don't need to spend time explaining what inertia is. If you're writing a non-fiction book about physicists for the general reader, however, you might want to. Don't forget that you can still tell stories with non-fiction, using narrative techniques like you would in fiction. Try to break your work into a beginning, middle and end with plot development, dialogue and recurring characters if possible. This makes the book more readable and makes the audience more comfortable with the information.