Every theme statement takes the form of a sentence, meaning it has a subject and a predicate. The subject is what the theme statement is about. A successful theme statement must have a subject that is relevant to the topic at hand.
If the subject is what the theme statement is about, the predicate is what the theme statement says about it. Choosing an interesting predicate can be difficult, but it is one of the most important factors in the success of your essay. Aim for an innovative angle on an old theory, a controversial assertion you believe is demonstrably accurate or a contrast or comparison that has not yet been drawn. The predicate of your theme statement defines your whole essay. Make sure it is a statement worth making.
A difficult task for beginning writers is to define an appropriate scope in their theme statements. Scope refers to both the range of ideas and the level of detail covered in your essay. If your scope is too broad, narrow it down by choosing a particular text, person or study as your context. To broaden a narrow scope, pick a contrasting idea or text and make the theme statement into a comparison of the two.
For a theme statement to be successful, the assertion it makes has to be one that can be shown to be either right or wrong. Your task as a writer is to convince your reader it is right. Your reader may or may not agree. But in either case, it should be a statement that is clear and concrete enough that there is something with which to agree or disagree. Vague claims, like "Ernest Hemingway was the best writer ever," are not measurable.
A successful theme statement should also make an objective claim that you can support with objective evidence. Even in subjective fields like literature, it is possible to support claims with objective observations on what an author writes or what historical trends influenced the writing. Subjective claims like "I think the government should stop raising taxes" are easy to fix; take off the "I think" and spend the essay defending your view. A claim like "I really enjoyed this story," however, is irrelevant to anyone outside yourself. Scrap it and pick something someone else could support just as well as you.