To write in plain style, we need to think about writing in a certain way (a way that might be called practical). Our assumptions about writing—what it should look and sound like, what it should accomplish, and so forth—not only manifest themselves in every sentence we write, but they dictate style. The quitchweed seed will produce quitchweed; the acorn gives rise to the oak. Planted in mud, they both die. And if our assumptions about writing are off the mark, then our skill with language is meaningless. We will write complex and impractical stuff.
Certain assumptions complicate style; others foster simplicity. Here are the ones that foster simplicity. Make them your own.