As I mentioned earlier, tacking on a minor, or second major in your senior year is usually a bad idea. But a minor in a related area can be a great addition to your education if it is well planned. As APS Preseident John T. Cacloppo wrote in September of last year, "Psychology is a Hub Science." This means that psychology as a field integrates with a lot of other scientific disciplines and an academic background in these synergistic fields can make a huge difference in your graduate school applications (and thus your career as a research scientist). Adding in a second field of study is something that ideally would happen early in your academic career, however it is a rare thing to meet a college Freshman that won't change her major at least once during her college career. Peter Vogt, a MonsterTRAK Career Coach, claims that 60% of college students will change their majors at least once before graduating. So what are some useful disciplines to tack on to a psychology major?
Biology (and other Life Sciences): This is a great addition for almost all expirimental graduate programs. Ultimately psychology reduces to the study of the brain, a massively complex biological organ. A good understanding of the physiological underpinings of psychology opens up a plethora of research programs and funding opportunities, so much so that I would say this the best minor you could possibly add. Okay, let me mitigate that by saying that if you are interested in clinical psychology or industrial/organizational (I/O) psychology, that biology isn't really necessary or even important.
English/Technical Writing: No matter what graduate program you go to, you will have to write a lot. From grant-writing and journal articles, to research proposals and your dissertation, you will have to write a lot. Having good language skills is a must in graduate school and you might as well get practice now. I'm not sure how much of a selling point this is on an application, but the skills you gain could be valuable.
Math: As a graduate student you will have to take quantitative statistics. If, for some strange reason, you are both good at stats and enjoy working with numbers, then you could make a career out of statistical analysis.
Computer Science, Philosophy, or Anthropology: These may be less intuitive choices, but for the field of cognitive science they make perfect sense. Cognitive science is a relatively new discipline that is the study of intelligence (or mind). Researching learning, memory, language, perception and artificial intelligence. The field is actually very interdisciplinary, and research seems to come from a lot of disparate sources. Outside of cognitive science these fields might still be useful, philosophy for example could be used in the emergent field of neuroethics, but I wouldn't count on them being a big draw for the majority of graduate programs.
Ultimately, adding a second discipline should be a carefully thought out decision. You should try to integrate the study of your minor (or second major) with psychology and allow enough time to complete both programs successfully. If you are a traditional student, then you are young and you have time, so even if it adds another year of undergraduate work, it may be well worth it in the long run. As always YMMV.