Some Practical Tips
As you go through your coursework, keep making notes of what you really find interesting, term papers you enjoy writing, research that has thrilled you, things you would like to know more about, theories that impress you. Any of these are possible broad areas for your research. Are there any questions that come to your mind, some problems you'd like to solve when you think of those areas? Do you have some ideas about how you can take things further in those areas? Keep making your notes and observations as you take courses and study for your comprehensives.
Before you even begin to seriously consider a particular subject of study, you must do an exhaustive literature survey. This means you must try to get to know the existing thought and literature in your areas of interest. Be sure you don't miss any work that's already been done or that is currently being done.
When the time comes, focus on one area and the broad subject matter of your dissertation. This is the time to ask yourself a few hard questions. Why is research in this subject needed? How can you ensure that the work you will do is new and fresh? How will it expand the world's knowledge of the subject matter? What value will it add to the existing literature on the subject? Will you be able to get enough resources, opportunities and support for your research? Will you be able to find enough previous work that will act as your foundation and give your work legitimacy?
Always, always pick an area of study that engages your passions! This is research that you will live with for a while. This is a topic you will be known in the scholarly world for, at least in the initial stages of your career. This is research you will (hopefully!) publish in some form or the other. If you're not in love with your topic, if it doesn't excite you, you will find it very difficult to complete your research and dissertation.
Listen to your committee members when they ask you to narrow it down! The dissertation is just the announcement of the beginning of your life as an academic and researcher, a preview of what you have in you to give to your field of study; it is not the sum of all your life's work. Most of us begin with too broad and ambitious a sweep as we set our dissertation goals. The idea is to keep your focus narrow and specific enough to be manageable and yet broad enough to be meaningful and substantial.
Once you've focussed on the parameters of your topic, ensure you make an outline and keep within those boundaries. This is easier said than done. The outline will become fluid and blur and you'll be tempted to go off at tangents. It is natural to veer away from predetermined paths and the whole point of doing valuable research is to not have preconceived ideas about the final discovery. At the same time, you do need an outline at all times, even if it is a flexible and changing one!
I guess what I'm saying is, don't overdo it, or it will take forever. You don't have to answer all the questions in your mind or present all the research you've done so far in your dissertation. You could pose them and deal with them in your further research. In fact, having those ideas up your sleeve will make you a better prospect as an employee and give you more papers to present and more publications to propose.
If my major professor hadn't stopped me in my ambitious tracks and if he hadn't forced me to severely restrict the scope of my initial ideas for my dissertation, I'd still be researching and writing it today!