What is the best course of action that I should take while earning my undergraduate degree, to prepare myself for law school?

I have had the privilege of serving on admissions committees in the past.  Volunteer work within legal services or even working at a law firm helps demonstrate a true interest in the law.  Your volunteer work should not come before your academic performance.  One of the first things most law schools will look at will be your grades and LSAT scores.  Make sure you maintain good study habits.  Law school involves a lot of writing and analysis.  Look at classes that will give you an opportunity to write and to think critically.  When it's time for you to take the LSAT don't just take it without preparing.  Save money up for a prep course so you can give yourself the best chance possible to do well.  Build relationships with your undergraduate professors or community leaders so when the time comes you can have strong letters of recommendation.  Can you hold positions of leadership without your grades suffering?  Working or volunteering in the area you say you're interested in; leadership skills; strong writing skills; good grades and a good LSAT score are all good goals to strive for as you prepare your journey towards law school.  Best wishes.

In addition to N Rutledge's answer, think about how you can specifically target these schools in your applications. Your application should NOT be the same to every school, you need to strategize!

I have a JD and an LLM (a Masters in law) and visiting the Dean of Admission for each school was the BEST thing I did during the application process. By doing this, I was able to tweak every essay/resume to best target each application. I was also able to decide which schools would not be for me, thus saving me a lot of money/heartache in the long run. I made an appointment to sit in on a class (this is very common), I signed up for a tour, and then I made an appointment with the Dean of Admission (this information can be found on the school's admissions website). If the Dean does not make appointments, try to make sure (s)he will not be out of town on the day of your scheduled visit. Sometimes they go out of town for recruiting events. But if the Dean is in town and does not take appointments, go to the Executive Assistant the morning of your visit and ask if the Dean has any available time that day for a drop-in appointment. This is not a work around, if the Dean of Admissions does not make appointments, the person corresponding with you about your visit will let you know about drop-in procedures. When you meet with the Dean be sure to have a few questions prepared (ie about the program, atmosphere, first-year expectations). Also, be sure to tell him/her what kind of law you are interested in and why. Give him/her some background information about yourself. When I did this, the Dean at each school gave me advice on how to target my essay and resume in order to have the best chance at an acceptance. They basically gave me the answers to the test! If you come prepared and show your interest, the Dean will help you. That's their job! Also, be sure to dress in a suit if you have one. Business professional never goes out of style.

For extra credit: Try to schedule a sit down with a professor. Ideally a professor who specializes in your field of interest or the class you just sat in on. If you have a connection to a professor through a friend, use that friend to help set up the appointment. And in your application NAME DROP. "When I met with Professor XYZ, we discussed the recent change in EPA regulations. Being interested in environmental law...." Word it to your liking, but name dropping shows your interest. The admissions board may even contact that professor about your meeting. If you can't schedule a meeting with a professor, stay after class (the one you visited) and have a quick chat with the professor.

Final Step: SEND THANK YOU LETTERS TO EVERYONE! The Dean of Admissions, the Professor who taught the class, the Professor you spoke with, etc. Always send one!