"Know your audience," or in other words, your readers. Teachers have preached this maxim for decades, but it bears repeating. Whether you are writing fiction, non-fiction, or school term papers, your writing will have the greatest effect only if you know your readers.
That seems obvious, doesn't it? Obvious it may be, but it's not always easy for writers to achieve.
"Knowing your audience" is actually a two-step process. The first step is deciding what readers you want to reach, and the second step is learning what those readers want to read.
The first step is the easiest for many writers. Part of the initial planning for your fiction or non-fiction piece should include deciding on the audience you want to reach. While this decision may not take a lot of time and doesn't need to be written, it is crucial, for it will determine the type of language (formal, informal, scientific, jargon), the complexity of the content, and even the style and tone of your writing. An article about black holes printed in
Boy's Life would be substantially different from an article about the same subject printed in
Scientific American.
The second step is sometimes unnecessary. A writer may already know his or her readers. A lifelong hunter would have no trouble writing an article for other hunters about the sport. On the other hand, this step can be a sizable stumbling block. Although we have all been young adults, an adult writer who decides to write a young adult novel needs to do some research - read current young adult literature and, if possible, talk to young adults.
An analogy: I've seen adults who have had no experience with young children sit down to play a game with a preschooler. They know enough to offer to play Candyland rather than Risk, but too often they treat the child like a little adult, expecting the child to understand directions and concepts too complicated for someone of that age when they should be tailoring their explanations and actions to the child's level of ability. Neither the child nor the adult is happy with the arrangement.
In Brief: Know your reader, and keep your reader in mind throughout the writing process.