The Writing Center at Harvard University states that a thesis statement must clearly illustrate the argument you intend to make and then explain the steps you will take in making it. Your thesis statement will not simply state a fact, as a fact requires no argument to persuade the reader. Avoid stating opinions, focusing instead on taking a position.
Review some of the available literature and research on your area of interest prior to attempting to write your thesis statement. This will give you an idea of potential angles and arguments you might make. Consider any controversial points you come across in your reading, as these present potential thesis statements. Make notes regarding contradictions, questions that arise for you or gaps in the available research.
Selecting the right topic is an important step in overcoming the challenge of writing a clear thesis statement. From your reading and your notes, select a topic that is broad enough to explore and debate but not so broad that you can't make your case fully in the space allotted for your assignment. For example, rising rates of obesity in the United States is too broad of an issue to explore and does not have the requisite argument. For your thesis statement, you might argue about one or two potential contributors that have led to that rise or challenge the explanations provided in the research.
Make your thesis statement clear and concise by avoiding flowery adjectives or words that are overused or misused, as well as abstract terms. The University of Washington suggests that a clearly written thesis has two important components: It is specific and divided. To achieve this, use as few words as possible to state your position, using specific rather then general terms. The term "divided" refers to clearly and succinctly stating why you have adopted this position.
Get a clearer thesis statement by applying a critical eye. Read and reread the statement, questioning whether it clearly articulates the argument you plan to make and considering whether you can effectively support that argument. Cut out unnecessary, abstract or confusing words. It helps to engage the second opinion of a peer to review it for clarity and style -- this proofreader may catch something you didn't and bring a fresh perspective.