How can I get a headstart on the LSAT?

One thing you might want to do (plus it's kinda fun) is look into Games Magazine. I suggest them because they have logic puzzles, as does the LSAT. Not identical of course but it can get you thinking that way, starting to wrap your head around the concepts. Here's another place for LSAT logic practice: http://www.west.net/~stewart/lwsample.htm

Another area is reading comprehension. Of course you have no idea what the topics will be in the actual test, but you can spend time reading and checking your comprehension. Say, the newspaper. Read the first and the last paragraph of an article (something not on the front page, maybe something on travel or fashion -- I'm suggesting something which isn't inherently super-memorable or that would give you a sense of urgency that a regular test question would not) and think up a few questions, and what you think the answers are. Then read the remainder of the article. How close were you?

Of course there are books, and eventually there are courses (I would not recommend starting one until you are a lot closer to the actual test date), but it's nice to study in a way that doesn't feel like real studying, particularly when you are this early.

I used to teach LSAT test prep (this was a good 15+ years ago) and one thing has not changed: there are a lot of people who cannot wrap their heads around the logic questions and freak out. Try not to be one of those folks and that's a good chunk of the battle.

Jes has great advice. I also used to teach LSAT test prep classes, and she is spot on.

I would emphasize working on timed problems. Lots of students have trouble finishing problems within a certain amount of time, especially the Logic problems; practice your setups until you can set-up a wide variety of logic puzzles within just a minute or a minute 30. At first you don't even have to bother finishing them on time, but eventually you'll want to be working problems very, very quickly.

Also - though I'm sure you are doing this - focus on your GPA if you are looking for a top-tier law school. You can directly affect that right now.

I work for a law school in California, I'd be happy to answer any questions you have.

Cycloptichorn

This is really the best advice for a college sophomore. There will be plenty of time to prep for the LSAT when you take it (probably in the summer after your junior year). In the meantime, keep your college grades up.