Several months ago, New York typographers Jonathan Hoefler & Tobias Frere-Jones of Hoefler & Frere-Jones, published this piece on mixing fonts. In it, they outline four tips for "navigating the typographic ocean". In short, they say your palette should be assembled with wit, energy, poise and dignity. Each of these points is thoughtfully explained with images and insightful descriptors.
If you hadn't been pondering the challenges of mixing fonts effectively before, you will after reading this brilliant little lesson from the pros.
Hoefler & Frere-Jones First Principle for Combining Fonts :
"Keep one thing consistant and let one thing vary."