I often find it difficult to structure 'compare and contrast' style questions. How do I do this?

One of the problems with questions like this, is that there is so much to talk about. Many students can fall into the trap of waffling and writing too much about a single point.
Using PEE paragraphs is one simple technique to improve the structure of your exam answers. PEE stands for Point, Evidence, Explain. For each paragraph, you begin by stating your point (how the texts are similar or difference). Next, you provide some evidence for the point you have made; you can do this by quoting the relevant parts of each text. Then finally, you explain the relevance of the similarity/difference. You can discuss why one extract is more persuasive/effective, or explain why there is a contrast (maybe the texts were written 50 years apart, and the attitudes towards women or religion are reflected).
This should improve the structure of your exam answers, and earn higher marks - as you will be writing in a more focused way and should be able to cover a breadth of points, without your level of analysis suffering as a result .