Mixing Fonts

Hoefler & Frere-Jones give techniques for how to combine fonts. Using their own, of course, they give four tips on combining the typefaces.
1. Use a palette with wit: use typefaces with complementary moods to evoke an upbeat, energetic air.
2. Use a palette with energy: mix typefaces from the same historical period whose families have different features.
3. Use a palette with poise: mix typefaces with a similar line quality if they offer different textures.
4. Use a palette with dignity: mix typefaces with similar proportions and give each a different role.
In a nutshell, use typefaces that have one thing in common and then experiment. Above is one example. Their variations came out beautifully.