This is an awesome write-up by Douglas Bonneville in Smashing Magazine:
Creating great typeface combinations is an art, not a science. Indeed, the beauty of typography has no borders. While there are no absolute rules to follow, it is crucial that you understand and apply some best practices when combining fonts in a design. When used with diligence and attention, these principles will always yield suitable results. Today we will take a close look at some the best practices for combining typefaces — as well as some blunders to avoid.
Yes, it's from three years ago but I recently needed to send it to someone and, while trying to look for it, realized that I hadn't actually blogged about it back then. So I'm talking about it now really just for completeness' sake :)