Now that I finally have grades, I'll add my two cents.
*Don't do substantive summer prep. Seriously, it's a waste of time. I know this is probably difficult to hear for some of you (considering the type of personality that TLS attracts), but your time is much, much better spent doing other things. Any other things, really. You have no idea how your professor is going to teach, or what areas of the law your professor is going to emphasize. And you'll pick up the material quickly enough during the semester.
*Use 0L summer to prepare for the 1L job search, law school generally, and law school exams. Do your best to determine what type of law you want to practice and in which geographic area/city you want to end up, and research the relevant employers. Make a spreadsheet of positions you're going to apply to for 1L summer. Prepare your resume and a cover letter template. Read Getting to Maybe. Read a book or two on the general law school experience. (I recommend Law School Confidential). And relax. Have a beer. You'll thank me later.
*Wait until a month or so before finals to begin outlining. If I would've started outlining immediately, my outlines probably would've been pretty shitty, and not very helpful. Outlining later helps you further absorb/remember the material from earlier in the semester, and it helps you "see the big picture."
*Practice tests. Do them. A lot of them.
*Brief cases. But don't go overboard. A few lines about the facts, a statement of the rule, and a few lines of analysis is just fine. This was useful in absorbing the material, on cold-calls, and (most importantly) in creating my outlines.
*Join a (active) student organization that relates to your career goals. This can help a lot in networking and in further exploring your career options.
*Keep a regular workout routine.
*Join a bar association in the geographic area you want to practice. They tend to be either free or for a nominal charge, and the networking opportunities can be fantastic. Speaking of which, network. Go to firm receptions and law school symposiums. Connect with attorneys in the region/practice area you're shooting for. Keep an active LinkedIn profile.
*Go to office hours. They're pretty useful, it turns out.
*You don't need to buy every supplement on the market. Supplements are useful primarily where you're having trouble grasping a concept, and sometimes to supplement your understanding of an issue. Buy a supplement if necessary, but it won't always be necessary. (My best class was one where I never consulted a supplement).
*Do more to really cater your exam answers (and exam prep) to the particular teaching/grading style of your professors. And find out what each professor's grading style is (this goes back to attending office hours). What was successful in my property class, for example, would've failed in my civil procedure class.
*People don't want to hear you constantly discuss/bitch about law school. Having other hobbies and interests and points of conversation goes a long way.
*Do more to maintain friendships outside of law school (!). Law school attracts certain types of personalities, and the law school experience itself can sometimes feel all-encompassing. You'll maintain your sanity by keeping those connections to the "outside world."
Edit: Wow, that was... lengthy.