One approach to this would be to compile a short list of quotations relevant to the primary theme or to secondary themes. You could analyze these in preparation for your exam and prepare to show how each points to the theme and elucidates the author's thematic message. An example would be the theme of artifice--that which is intentional in cleverly deceiving through devices or tricks--and it's relevance. A quotation you might tie in with this is this quote related to artifice: "commend me to a mask of silliness and a pair of sharp eyes for my own interest under it."
Another approach to selecting important quotations is to focus on principle characters and select quotations that show their (1) characterization and their (2) character development. Characterization is the traits and psychological qualities an author describes a character as having or describes them through exposing. Character development is how the character changes as a result of the conflict and experiences the character undergoes. For example, you might illustrate Captain jack Absolutes characterization with the quotation that illustrates his own idea of his psychological motivation that, as he says, excludes the "doubts, fears, hopes, wishes, and all the flimsy furniture of a country miss’s brains."