Find a topic that suits your research interests but also unique. If you're interested in researching the positive and negative impact African-American mayors have had in the cities they presided over throughout past twenty years, read some books and scholarly articles from other authors on this subject; then, look for a way you can add a different angle to this topic based on what you read.
Choose a topic that will relate to your future career. For example, if you plan to work as a political analyst for a local TV station, choose a topic that pertains to the way the media covers political subjects on a daily basis. If you plan to become a political science professor, choose a topic that focuses on effective ways that university political science departments can prepare their students for political or legal careers.
Discuss your research interests with your advisor. If you want to research the history of America's first ladies over the past fifty years and how their personalities and political beliefs shaped the lives and decisions of their husbands while in office, ask your advisor how you can turn these interests into a well-researched and interesting dissertation.
Choose a topic that you can handle. If you're interested in writing a dissertation about the politics of the Civil War and its effect on Southern women, find a niche within this topic so you won't take too long to complete the dissertation. Instead of focusing on the effects of the Civil War on Southern women in general, focus your topic on the effects of the Civil War on slave women and free women of color.
Choose a topic you have some expertise in. If you've already written some articles and research papers on the role that Latin American immigration has played in American public policy over the past thirty years, choose a unique topic that builds on your expertise in immigration issues.