My introduction to any song I sing (and I usually sing a cappella, so it's not just something to kill time while I tune)--my introductions are very much dependent on the context--where I am singing the song, for whom I am singing it, the song(s) that preceded it (or occasionally, the ones that will follow it)--My usual introductions for my Anglo-American repertoire include information about where the song comes from, mainly because I sing in a traditional crowd where such information is expected and appreciated. My foreign language songs are introduced with a translation, often just of a line or two, but enough to give the audience a sense of what I'm about to sing.
Some songs I sing absolutely require introduction. For example, if I sing "Lunatic Asylum" (from Tennessee) without an introduction, someone will giggle at the first mention of words "lunatic asylum", and yet properly introduced the song can be a very poignant artifact of a person wrongfully committed and trying to make sense of their incarceration. I can set up a version of Lord Randall (John Randolph) as a tale of a boy not wishing to share details of a love-gone-wrong with his mother, rather than just a boy-dies-of food-poisoning song.
I'm not beyond using humor. I love to put a local spin on at least some songs I sing, showing how they could relate to local history, even if the song itself comes from another country and culture. I pay attention to the length of my intro, and watch the audience for subtle cues and clues as to whether they are into more brevity or greater detail. The only hard and fast rule is that there are no hard and fast rules--good song set-up is an art, every bit as complex as presenting the song itself.
--Charlie Baum