This is a great question! Jane offered a lot of great advice. I would go a step further and say that while you are in undergrad that you volunteer, or take on internships in writing. The year book, a blog for the school, or even pamphlets as they relate to psychology. Go ahead and get more than APA formatting under your belt as well. AP and Chicago styles are important in journalism.
You could even team up with a faculty member who loves doing research but doesn't like writing and get yourself a peer-reviewed publication before you graduate.
You can major in journalism and minor in psychology if you like. This way, you still have all of the grounding of a psychology degree with the skills you will learn through journalism.
All the best,