Use fonts that complement each other.
via The Principles of Design: Font Pairing — The Daily Post
This is a good reference for font pairing, although I hate using the word font as a designer, even though it’s accepted. I think typeface was drilled into me in design school by an instructor who despised it. But I digress. Check out what he has to say regarding why us crazy designers choose the typefaces we choose and check out his other topics on the Principles of Design. Good stuff!