Great question since there is really no secret formula. However, there are courses to help. Law School is designed to make you think. Yes, there is black letter law, and you either know it or look it up for accuracy or you don't. But it's also important to train your brain to think logically and consider multiple scenarios.
Developing your common sense and strengthening your argumentative reasoning is key. Classes like philosophy, political science, and sociology do just that. You also want to take classes that get your comfortable with presentation. Introductory acting courses, and taking on leadership roles in student organizations really help to challenge your thinking and make you comfortable and confident in the spotlight.
Lastly, writing is very important. This is where you craft arguments and support your reasoning with proof. Many courses in liberal arts will have research paper midterms and (although sometimes annoying) it provides amazing practice for law school.
Hope this helps!