Ask the students to draft a list of what they find important about their topic. For example, a student who's writing about a novel should list essential themes, motifs and plot in the book. A student who's writing about an event in history should outline the basics of the event and list the important players in the action, as well as the impact of the event on history.
Point out debatable ideas in the students' lists. Theses contain debatable ideas. The student will spend the entirety of the paper proving that debatable idea. For example, a student could not write a thesis on a point such as "John Wilkes Booth shot Abraham Lincoln." He could, however, write a paper about the Lincoln assassination that surrounds an idea that historians debate. An example is, "The U.S. government executed Mary Surratt for her role in the Lincoln assassination; however, she was innocent."
Explain to students the importance of debate when it comes to a compelling, educated paper. You can read a paper that recounts the same dry, unbiased facts a hundred times over. However, papers that explain upfront what they hope to add to the study of a subject make for a more compelling read.
Stress the importance of proof. Students can begin with a "working thesis" that they adjust over time as they perform their research. It's not enough to make a debatable claim; students must do their best to prove their claims with facts in their papers. Ask students to write down a working thesis and to highlight the themes, motifs, facts and persons involved on their lists that can support their argument. They may find that they think of another argument instead; they could even decide to write a paper on the opinion opposite the one in their working thesis.