It is very important to realize that finding a good match for graduate school is very different from choosing a college for your undergraduate degree. In fact, finding a good graduate school match requires considerably more effort – plan ahead and realize that this will take a great deal of time. The crucial realization is that you are seeking a specific program (not a school) that not only matches your interests, but where the faculty are active contributors to the field. This is especially true for Ph.D. programs, where the relationship between the student and primary mentor is extremely important – think of graduate school as being a bit more like being an apprentice to a specific mentor (rather than a place where course-work is the central activity). In Masters-level programs, the difference between undergraduate and graduate education is not as stark.
The next step is to identify the specific graduate programs that best match your interests. For example, if you decide to pursue a Ph.D. in clinical psychology, your next step is to identify which of the many programs best provides a fit with your specific interests in this field. Finding a good match will depend primarily on the specific topics that most interest you and the faculty members' areas of interest. For example, if you wish to conduct research on integrating different approaches to conducting psychotherapy (known as "psychotherapy integration"), you would want to find out which faculty members are prominent in this area of study and apply to the graduate programs where they teach. Using this example, some of the most prominent researchers in the area of psychotherapy integration can be found by looking at the membership of this area's main professional organization, the Society for the Exploration of Psychotherapy Integration (SEPI): http://cyberpsych.org/sepi. If you click on the 5th link down on the left, "Books by SEPI Members" you'll see some excellent books by leaders in the field. Look at the authors of these books, find out the universities at which they teach, and then check on the corresponding graduate programs. Buy some of the books that interest you most and see who publishes a lot in the field, who is often cited by other people in the field, and apply to the programs where you now know you could get training in psychotherapy integration by a leader in the field. Of course, with the internet and search engines, it is now easier than ever before to find out "who's who" in a given area of specialization and at which universities these people teach.
In any field you are applying to, it is important to find out who the active contributors are, not just who has already achieved prominence (sometimes prominent people are at the tail end of their careers and are not closely involved with graduate students or publishing new research – hitching your star to a person at this stage may turn out to be a bad idea).
One of the most important issues in selecting a graduate school in psychology is the "mentor" with whom you will work. Most graduate programs in psychology use a mentor approach where graduate students work very closely with a single mentor or perhaps two mentors. The mentor supervises research, serves as advisor on the best courses to take given your interests, and shows you the "ins and outs" of how to conduct and write publishable research. If you have a mentor that you work well with, then this will be of enormous help as you go through the rigors of graduate school. On the other hand, if you have a mentor who does not provide much support, it can make graduate school that much more difficult. Therefore, one key goal is not only to pick a program that serves as a good match with your interests but also to select a mentor who you have every reason to believe you will work well with. One of the best markers of a good mentor is a professor who has a history of graduate students who have had fruitful careers. When you get to the point of interviewing at a university, you will have ample opportunity to find out what the current graduate students know of each faculty member's history with students. You also would be well advised to speak with your current professors to learn as much as you can about a prospective mentor. Sometimes the name of the mentor won't ring a bell but sometimes you will get very useful information from your professors. It is perfectly reasonable for you to ask around about a potential mentor. Any mentor who objects to you finding out what you can about the mentor is not likely to be someone with whom you would wish to work during graduate school.
As you research potential mentors, also research which programs are considered to be excellent given your interests. For example, if you wish to specialize in child clinical psychology, Vanderbilt University has an excellent program (as well as excellent mentors). Also, check out the person's recent research (e.g., do a PsycInfo search). If you do not like the research this person does, you are unlikely to have a good graduate school experience.
Acceptance into many graduate programs depends on a specific faculty member choosing you to work with (i.e., deciding that he or she wants to take you on as a student). As a result, it is important to research the faculty at a specific program and to tailor your application to them – you need to convince one or more members of the program that (because of shared interests) they will want to work with you. Make sure you know what the faculty members are currently interested in (they will not be impressed by an applicant who cites their research from 10 years ago when they have long since moved on to something new). Contacting the faculty members you are interested in, noting your interest and perhaps asking an informed question about their work, is a good strategy (email makes this easy). You do not want to be a nuisance, but to initiate contact and see what happens. As well as letting the faculty member know who you are, you may find out some important information. For example, the faculty member may reveal that he or she is not taking on students for the next year, but will be the year after next (which might inform your own decision about whether to go directly to graduate school or take a year off). Or you might find out that the faculty member is taking a job at a different school, leading you to apply there instead.
As you build up your list of potential programs that match your interests, keep in mind the following rule of thumb: Apply to a lot of programs (especially if you are applying to counseling or clinical Ph.D. programs, as many as 15-20 depending on how difficult they are to gain admission). For these areas divide the schools into three categories, lower, middle, and higher tier and apply to at least 5 in each category. None of the schools are easy to get into but some are more difficult than others; these categories, therefore, simply reflect their relative difficulty of obtaining admission. For other areas, it is appropriate to apply to fewer programs (say 6-10). Get advice from one of your professors in the area.
Once you have narrowed your list to the schools to which you will apply, then the process of obtaining applications and completing them is simple at some levels (e.g., getting the application on line) difficult at other levels (e.g., writing your "statement of purpose" and doing "the interview"). But the successful applicant is likely to go beyond this by doing such things as contacting faculty members with whom they wish to work.