Start while you are still a college student. During college it is important that you are an exemplary student. You may not be able to get straight A's, but be an engaged student. This will get the attention of your professors. The more your professors respect you, the better their letters of recommendation will be.
Get involved in research or other activities expected of graduate students. Most graduate programs require that students become involved in various independent research projects. Graduate school applicants that were involved in research activities during college are often given more consideration than those without research experience. If you can publish any of your research in a scholarly journal you will be even further ahead of the other applicants.
Begin studying immediately for the Graduate Record Examination (GRE). As soon as possible begin studying for the GRE (or other entrance exam). Your score on the GRE can be one of the most significant factors in whether you are admitted into a graduate school.
As a college student, be active in your discipline' s department. This includes activities such as being a member (or officer)of your department's club (e.g. psychology club), or taking a work study job in the department.
Begin your search for potential graduate schools early. The admission process is very time consuming. The earlier you start locating a school, the more time you will have to fine tune your application materials.
Choose schools that most closely match your professional goals and specific interests. There are typically several graduate programs to choose from for any discipline. Be sure to evaluate each program's specific areas of emphasis, and the research interests of the professors for compatibility. Applying to a school that is not compatible to your interests is a disservice to you and lowers your chances of admission.