A good thesis statement comes only after exploratory investigation of a subject. That is, you cannot simply say "Horror films are violent" without having watched a few or read some material on the subject. Visit your local library, read about the subject online or consult experts on the matter. Even if you think you have your position before you begin your research, your thesis might actually change several times throughout the research process depending on the information you find. Develop your interpretation of the subject. Define the subject by comparing it to a similar one, for example Greek and Roman art. This comparison could even become the subject of your thesis statement.
Whether your research paper is particularly argumentative or not, an effective thesis will take a stance on an issue. If you are not positing a dichotomy between two ideas, you should deliver a thorough interpretation of the subject. For example, if the subject matter is American literature, your thesis might be "Though it varies in tone and style, American literature borrows heavily from pivotal English works." This sentence does not present an "either/or" statement; rather it introduces the stance you will be taking throughout the document.
A strong thesis statement should address the subject at hand clearly and in detail. Do not pad the thesis statement with convoluted jargon that takes away from the impact of the essay, at least in the introduction. Make the subject of your thesis statement as specific as possible so that the reader is clear on the subject. For example, you should address 19th-century inventors from the southern United States, not just historic inventors as a whole. Keep in mind that the more specific your statement is, the more information and research you will need to support it.
Keep your thesis statement one to two sentences in length. The introduction of the research paper or project will introduce the ideas leading up to the thesis statement, so make the wording of the statement as concise and effective as possible. It should make the argument clear to the reader. For most academic and professional essays, avoid filler words such as "I feel" or "I believe." The reader already knows it is your opinion because you are the one writing the essay.