Choose a field you'd like to pursue. Take classes in that field and begin tailoring your class papers and course projects to your chosen field. If you have a Senior Project requirement for graduation such as a research paper or culminating project, gear your work toward a finished product that you could submit as a writing sample in your graduate school application. Make sure that your writing sample has been proof-read and edited at least three times. Develop language skills in a second language.
Bilingual applicants have a great advantage.
The best preparation is to become very familiar with the format of the questions on the exam so that you won't end up using precious exam time to understand what the questions are asking. Research graduate programs and potential mentors. Find the top 5-10 programs that match your goals and objectives and research faculty members in those Departments. Show how you are perfect for the Department and the Department is perfect for you. This is an important point to make in your application.