A popular adage states that high school teaches you what to think, while college teaches you how to think. Reflecting on my experience in the PSY 1001 course, the latter definitely rings true.
One concept from the class that I will certifiably remember the most in five years and beyond is the framework for scientific thinking and skepticism we learned at the very beginning of the semester. In a few years, I may not be able to recount all the details of the sleep cycle, elaborate on each theory of cognitive development, or describe the anatomy of the central nervous system. However, I will forever apply the logic of the six principles of scientific thinking when presented with new assertions and information.
The seven words "rival causes falsify repeated claims, Mr. Occam," is more than just a mnemonic device that helped me through exams. It equipped me with a new basis for how to think critically about various claims and arguments. For example, how often have we run into situations in which we're presented with a claim that's literally incapable of being disproved (falsifiability)? Like how Santa Claus and the Tooth Fairy "magically" appear only when we're sleeping.
One of the most important psychological principles, in my opinion, is that correlation isn't causation. For example, my friend once humorously asserted the following: "I have skin. Potatoes have skin. Therefore, I am a potato."
While the specifics of each subject in psychology are worth knowing, nothing will prove more useful or more applicable to me than the principles of scientific thinking. I haven't just learned what to think, I've learned how to think.