I. Get to know at least some of your professors outside of class.
As Orin suggests, it can help you understand the class assignments better. But just as important, it will be a big help when you need to ask them for recommendations for jobs or clerkships. If all I know about a student is that he or she got a good grade in my class and did well in class discussion, I can't write a recommendation that will say much beyond what the employer can learn by reading the student's transcript.
Sadly, there are some professors who still don't realize that seeing students outside of class is part of the job. But more often than not, talking to profs will be easier than you think, because most academics love to talk about their subject! If they didn't, they probably wouldn't be in academia in the first place.
One particularly good way to get to know professors is to work as a research assistant. It certainly worked well for me.
II. Do the reading.
I'm not a believer in the theory that law school classes are vastly different from other types of classes, or that mastering legal materials is some kind arcane art that is unlike anything else you will ever study. But one way in which law school classes do differ from many undergraduate classes is that it's much harder to "wing it" without doing the reading. Legal issues often turn on fine distinctions buried in judicial precedents or in the text of statutes. It's hard to learn these things if you didn't do the reading.
III. Get to know the other students.
Many of your classmates are likely to end up working in the same field of law as you do. They can be extremely useful connections. And if you're lucky, they might turn out to be interesting people as well. This is one of the things that I neglected when I was in law school. It was a major mistake. So do as I say, not as I did!
IV. Do extracurricular activities.
Most law schools have a variety of activities for students outside the classroom, including numerous clinics and student organizations such as the conservative/libertarian Federalist Society and the liberal American Constitution Society. Getting involved in these will help you learn more about the law, and can also lead to important career benefits. You may learn about an area of law that will become the focus of your career, and you can also develop useful contacts that will help you later.
If you are at all interested in constitutional law or in public policy, I especially recommend the Fed Soc and the ACS (established as the left's answer to Fed Soc). But there are lots of other groups catering to a wide range of interests. If you go through law school doing nothing but taking classes and studying, it will probably be a mistake - both educationally and career-wise.