Martin Carthy is my hero of song introductions. He often tells the whole plot of a long ballad -- interjecting humo[u]rous anachranisms, like "and he stopped off for a video on the way home" -- and you'd think that telling the whole story would ruin your enjoyment of the song. But it doesn't because 1) He gives you context that we don't know anymore that our ancestors did when they heard the song, so you understand and appreciate the song more than you would have if he hadn't talked about it; 2) He sometimes tells you what or whom to watch out for, which you might miss in the 14th verse because your attention might wander -- for example, pointing out that the wife of the Lochmaben harper only has a line or two, but she's the one who's really smart and saves the day, so listen for what she says; and 3) he's ususally very amusing.

I haven't quite got the hang of it -- I think I tend to go on more than is necessary sometimes. But sometimes I've figured out a single line that sets up a song perfectly. Problem is, you can't always use the same line. Jokes do stale faster than the songs.