This "guess the author/date" warm-up can be done in a light-hearted fashion if people feel inhibited. It might also help to show that even the experts disagree on poems. (e.g. Wordsworth's "Lines Composed ..." has attracted extremes of comments. I think it's hopeless).
I tell people what I really think about the poem afterwards. I'm wrong at least as often as they are.
Once, each person was given at random a piece of paper showing what aspect of the poem they should concentrate on