Your topic sentences let the reader know what you will argue in each paragraph, and how your paragraphs support your thesis. Topic sentences should not be statements of fact, but debatable assertions. Ideally if someone were to just read the topic sentences for your essay, they'd still understand your basic argument, even if they lacked the evidence to support it. If your argument isn't clear, take the last draft you wrote and "reverse outline" your paper: that is, on a sheet of paper write out the thesis for your paper and the main argument for each paragraph. These arguments should be included in your topic sentences. The argument in your topic sentences should be as specific as possible.
Here are some sample topic sentences:
- "The plays are each other's negatives in one other important way." (Ok--this topic sentence makes an argument, but it is still unnecessarily vague. What is that "other important way"?).
Revision: In both the Oresteia and Oedipus Rex, knowledge is a curse; however, Cassandra, in her forward moving play, is cursed by the knowledge of coming events, whereas Oedipus, in his play of regression, is cursed by knowledge of the past. (Better--now I know what the "important way" was, even if I don't know why it was important)
Even Better Revision: The Oresteia and Oedipus Rex display fifth-century Athenians' crises over truth: for both Cassandra and Oedipus knowledge is a curse, even if one knows the truth before it happens while the other must wait to have it revealed after the fact. (Ah hah! I know both "the other important way" in which they are inversions of one another and I know why I care.)
- Both stories has civil crisis, however both treat them very differently with respect to the roles of the leaders and their reporting of the facts. (This topic sentence is ok. It indicates that the two stories differ, but it doesn't tell us the specifics of how they differ or the significance of that difference.)
Revision: Thucydides' greater interest in the agency of civic leaders points to the rise of human versus divine agency in Athenian society. (This topic is more specific about what makes Thucydides innovative, and explains why we should care about his innovations.)
For sample exercises on topic sentences see Diana Hacker's A Writer's Reference.