Here's a principle that serves me well: Begin by connecting with the reader, with what they already know. Then take them somewhere new.
Begin each sentence with something the reader already believes, or knows, or cares about. Put new information at the end. Then you can start the next sentence so that it connects with this new information (which they now know), and take them one more step.
The flow of your content won't always be so linear. So you can start a sentence with:
- An idea that the reader knows from experience, or other sources.
- Some connection to the idea with which you ended the previous sentence. Maybe you'll add to it, or restate it, or highlight an implication, or zoom in on a single detail.
- Reconnecting with ideas you introduced before the previous sentence.
The same idea works for paragraphs, sections, chapters, and entire essays.
Connect, and take a small step. Connect again, and take another step.
I learned this principle from Joseph M. Williams's enormously helpful book Style. The subtitle is "Lessons in Clarity and Grace." I recommend it highly.