Read extensively in the specific area in which you intend to write your dissertation. Because you have to build upon the existing knowledge, you need to have a thorough working knowledge of your topic. Doctoral dissertation topics have become fairly narrow and specialized in scope because of the amount of scholars constantly contributing to the field through the writing of books, articles and other dissertations.
Analyze the literature in your field of study. Look for flaws in the methodology of other scholars or for flaws in their logic. Scholars will sometimes write dissertations in which the conclusion does not necessarily follow from data presented within the thesis. Or, in some cases, you may uncover data that sheds new light on the topic and alters it significantly. These types of alterations to the scholarly literature are the types of topics on which many scholars build their entire careers and reputations.
Think of your dissertation as your niche within your field of study. For instance, American history students cannot simply write an overview of a period in American history because the topic is too broad. If you are going to write about a specific period, then you will need to narrow the topic and the range of time, usually one to 10 years at most. Within that range of time you would still need to narrow the topic to an area like labor relations or American religion. These topics would also have to be narrowed to a specific problem, event or individual.
Develop a solution to a problem. In some areas of academia, like education or psychology, the dissertation is developed with a specific problem in mind. Occasionally these research topics are developed in relation to clinical research developed by the doctoral student. For instance, if in your reading you discovered that a psychological study involving autistic children had proved to be inconclusive because of flawed or incomplete statistical data, you could conduct a similar study introducing the new variables that would complete the study.