I always recommend picturing your readers as completely as possible, using the "PACK" acronym: What's their personality? What's their attitude? What circumstances are they in? What's their knowledge of the subject? But what if you're writing to someone you don't know?
Daphne Gray-Grant, in her newsletter Power Writing, has a great solution: write for just one reader. She recommends identifying the characteristics of a "core reader," then giving him or her a specific identity. She writes,
The biggest benefit of creating imaginary people . . . is that you stop thinking about yourself. Instead of focusing on your own needs and problems, you're suddenly thinking about what the client or the reader cares about. And by giving your core reader a name and a face you transform him or her from an anonymous mass into a real human being.
This week, give Daphne's advice a try. When you're writing to a reader or readers you don't know personally, create a core reader, with his or her own name, personality, attitude, and circumstances.