What is the best way to prepare for an unseen poem on an exam?

The best way to prepare for an unseen poem is to practice close analysis before hand. There are certain things that will always be applicable for poetry. You're going to want to talk about the structure of the poem; The verse, the stanzas, the rhyme scheme, anything that's interesting about the way the poem is laid out. If the poem is in a specific form, such as a sonnet or haiku that is something that will always be really good to talk about. In the same way, there will always be something significant about the imagery and language of poetry. Look for the metaphors, the hyperbole or the juxtapositions. Try to find if there is any significant semantic field to highlight. spend the first five minutes going through the poem and highlighting any and all interesting things you can find. Once you've done that, and only then, start stringing these things together to get a whole cohesive picture of the poem and start constructing your analysis. The best way to get used to picking these things out is to try it on poetry you might already be familiar with. The more you practice the easier it will be.