Q: Any tips/rules of thumb for voicing non-diatonic passing chords and/or temporary key changes to make them really uh....good?
A: it's all about songs. get to know songs like people, make friends with each one of them. These things you're talking about, it's just the same as traits in people. you never know what might make someone the magical person they are. but the more people you know, the more you might be able to dream up and invent these things in song, seemingly out of nowhere