Choosing one type is still comparatively easy. It becomes more difficult with the combination of several types. Here some tips:
When you use types that differ only in details (like a Garamond beneath a Times), there comes up a certain disharmony. On first sight, most of the readers won't make out any difference but they feel that there »something is not right«.
It is similar with gothic types (serifless linear roman), as like Helvetica together with Futura.
So choose two typefaces that have an obviously different look, but that go together in harmony (how about Times and Futura?).
But it is also important, that you don't overload your documents with typefaces – normally two types are sufficient for shaping text.