How should I effectively revise for seeing an unseen text?

The best way is to give yourself tasks which focus on the assessment objectives. Find anything you haven't read before and go through it, annotating any techniques you notice and making notes on any theme or device which you find interesting. Once you have read it through and annotated it, collect your initial observations into a brief outline of a point, evidence, explanation paragraph. At this point, you can either use what you have so far to compare to another text and/or push further into your reading and analysis of the text you have found.