What one line in your prompt takes Claude's writing from 5/10 to 9/10?

A user on Reddit's dev community is seeking advice on how to improve Claude's writing to sound more natural and less like AI. They use Claude for work emails and writing, but find the drafts to be merely "okay" and not reflective of their personal style. They are looking for specific prompt lines or setup strategies that can elevate Claude's output from a 5/10 to a 9/10, asking for any simple or complex advice that helps make the AI-generated text indistinguishable from human writing.