I listened to Andrew Clement’s book Frindle in the car with my kids during vacation. We all enjoyed it very much.
One writing technique that stood out to me the most was the way he described the eyes of the teacher in the story.
Each time he described an important interaction with Mrs. Granger, he mentioned her eyes. Her eyes always said something about what she was thinking even if she said something different. When you think about it, the eyes of a teacher do say a lot. Clements took that physical characteristic and accentuated it in describing her in each scene.
When you develop your protagonist, antagonist, and even secondary characters in your story, pick one physical characteristic that you can repeatedly describe in new ways that shows a character’s motives and development.