What makes good analysis of an unseen passage?

In terms of general advice, when faced with an extract it is best to go through it line by line, reading it slowly. Try to analyse each line of the extract if possible, then select your best analysis for your answer. If it is a text with which you are familiar with, demonstrate that to the examiner- briefly place the extract into the context of the text. Analysis of any piece of literature should be divided into two main sections: language analysis and structural analysis. Both these terms seem daunting, but there are methods which help break down these broad terms. For language analysis, an easy approach is to envision the "language analysis triangle." Essentially, within this structure, there are 3 tiers, each ascending platform more difficult than the last. So, for example, the bottom tier includes quite simplistic language analysis techniques, such as metaphor, simile, onomatopoeia, imagery to name a few. The next tier contains a few more sophisticated concepts like juxtaposition, repetition, pathetic fallacy, lexical field etc. The final tier contains the most complex language analysis, with devices such as metonymy, neologism, bathos, pathos, symbolism, irony. Indeed, to get the best marks I would argue that your answer should include devices from each tier.
You can also apply the "triangle" technique to structural analysis too. The bottom tier initially deals with the form of the extract- is it in verse or prose? Is it dialogue? If it is dialogue, who is speaking the most, and what does this tell you? If it is a poem/verse, what kind of poem is it, and how do you know this? Quite simplistic analysis. The next level includes analysis of the rhyming scheme and rhythm of the language, if there is one; I would argue that this can be placed under structure or language analysis. The final tier forces you to look simply at the lines and format of the extract on the page: it sounds simplistic, but can often be quite difficult. Try and notice any devices such as enjambment, or the length of stanzas and lines; this technique might actually help you identify what type of literature you are dealing with. Remember to always support your findings with evidence from the extract- I hope this was useful, and good luck!