Much of this advice applies to students wanting to go on in any subfield of psychology, but it was written for those wanting to become psychotherapists (the hardest of all subfields in which to gain entrance). The career path to becoming a psychotherapist is a long one, so it's a good idea to gain as much experience as you can before committting to it. Psychotherapy training is competitive. Clinical psychology PhD programs in the U.S. are the most difficult programs to which to gain entrance (harder, even, than medical school or law school). Although professional (non-University based) programs have popped up in the last few decades and are easier to get into than are Universities, they are very expensive and their faculty tend to be more transient.

As an undergraduate, there are lots of ways to distinguish yourself to enhance your application to graduate school.

Undergraduate years. First, you'll need to earn a bachelor's degree (B.A.) in psychology. During that time:

• Strive to take courses from a variety of disciplines. Many B.A. students are overeducated in psychology, and undereducated in everything else! Take courses from biology, neurology, anthropology, sociology, literature, history, and art. These will enrich you, and will inform your studies later on. Take the minimum number of psychology courses required to graduate.
• Work hard to achieve your best performance, especially in the psychology courses you take. In order to get into a good graduate school, you'll need a very high GPA. Use study techniques like SQ4R (described in most introductory psychology textbooks). Treat school like a job: commit yourself to spending 40 hours per week studying and attending classes and discussion sections. When you reach 40, stop, but don't stop until you reach 40. (Universities design their courses so that students are supposed to spend three hours per week on each unit of credit.)
• Take Hard Classes. Gear yourself toward statistics, advanced statistics, advanced research methods, neuropsychology, neurobiology, and so on. The students you'll be competing with to gain entrance to grad school will have done so. This also shows you are willing to accept a challenge, and can excel at understanding and applying difficult or complex concepts.
• Volunteer at a nonprofit agency or school for as many hours per week as you can reasonably do so, certainly during the summer. These experiences help you to develop as an individual and member of society. They also help you to decide whether or not psychotherapy is really the career path for you! Some students think they want to be therapists because they want to understand themselves. That is a fine motive initially, but it won't sustain you through graduate school. Seek out two or three different volunteer experiences during your undergraduate years. Most schools have field study or other volunteer programs through which you can achieve academic credit, too. Then, when you apply to graduate school, the admissions officers will know that your decision is based on real-world experience.
• Get research experience. This is extremely important. Check with your psychology department's advising office, and learn the protocol for finding a professor who is doing psychological research of any kind. Although psychotherapy research would be preferable, it can be hard to find a project to join. Any kind of research will give you real-life experience in what it means to be a psychology researcher. Some schools provide academic credit for you as a research assistant. If your school has no such opportunity, construct your own research project. Make sure to approach a professor to get academic support, and your school's Protection for Human Subjects committee to have your design approved before starting the project.
• Teach a seminar. Some schools allow undergraduates to teach their own seminars in their senior year.
• Apply for undergraduate awards.
• Get to know three or four professors well so that they will agree to write you letters of recommendation. Tenured professors' letters have the most clout. Also ensure you will have a letter or two from volunteer work supervisors. Visit these people in their office hours throughout your undergraduate years. Approach them at least six months in advance of needing a letter from them.
• Write a senior thesis. Some schools offer that opportunity to all of their students; others offer it to students who have done very well in their first three years.
• Develop extracurricular interests: your hobbies, membership in clubs, and travelling all say something about you. Most graduate schools like students to have had firsthand experience with other cultures, too.
• Have fun!