Focus on one idea.
Let’s start with a definition. An article is a short piece that focuses on one idea. A chapter is a short piece that focuses on one idea. In the chapter of a novel, several things may happen but the chapter has a single purpose and stays with it. It's also true with a chapter of a nonfiction book in which you may explain five ways to avoid a heart attack. But all five methods stay with the same theme.
Here’s an easy way to see how this works. Pick out two magazines. (I suggest you avoid ezines. Many of them are badly written and poorly edited.) Read three articles in each magazine.
As you read, ask yourself: What is the one point the author makes? The title should help. If it’s a how-to article called "Three Ways to Lose Weight," that points the direction. If it’s something such as "The Day Dad Cried," everything in that piece needs to point to a single, poignant event with no distracting information about where Dad lived when he was fifteen (unless it’s relevant) or the fact that he went to school with Brad Pitt's mother's younger brother.
Open a novel at the beginning of any chapter and the principle works. If we look at books from 100 years ago, they often had a table of contents for fiction that told readers what they were about to read in each chapter.
"It's not portable," an editor said about an article I wrote 30 years ago. He meant it wasn't focused. You don't want to get a similar message, do you?