Every man is a borrower and a mimic, life is theatrical and literature a quotation.
- Ralph Waldo Emerson
A fun approach I like to use (sometimes unconsciously, sometimes deliberately) is to have an unofficial objective of having three delightful details in a design — a humorous error message, a helpful hint at a difficult juncture, a way to skip a step in a process, etc. The important thing here, as Jeff suggests, is to do it at an unexpected time, almost as a kind of surprise, just so it has an extra emotional impact on the user.
Random Acts of Design Kindness