The best advice I received early on was fairly simple. If you're still in an undergraduate program, work on getting the best possible grades you can muster in your remaining courses. Write a strong senior thesis or craft a shorter writing sample for publication in a small journal or magazine related to your field. Also, continue to cultivate strong relationships with a handful of faculty members by visiting them in office hours. Study for and take the standardized test required for admission to most of the programs in your field (GRE, MCAT, GMAT or LSAT depending on your field).
When you visit the aforementioned faculty in their office hours explain to them your major interests and reasons for applying to graduate school. Pick their brain on who the big players in the field are, where do they teach, where did they attend graduate school? Do all the best people in your field teach at Virginia? Minnesota? Berkeley? Harvard? Check out U.S. News and World Report's Best Graduate Schools issue and try to get a field for the important schools in your field. Many scholars will disagree with how US News ranks graduate programs, but it offers a nice starting point. Start compiling a list with brief notes describing why they made it on the list (for example, Harvard - ranked no. 2 overall in general field, no. 4 in my proposed area, and has faculty John Doe and Jane Brilliant working on history of women living in urban high rises).
Build up this list to the point where it is nearly unwieldy. In conversations I had with faculty at my undergraduate institution and others in the field whom I had met, I came up with a list of about 25-30 schools.
In our next post, I will talk about paring down that list to a more reasonable list of schools to apply.