The following is an excerpt from the book The Ivey Guide to Law School
Admissions by Anna Ivey.
How to Pick Your Recommenders
Avoid jumping all over the first person who offers to write you a "great recommendation." Recommendation writing is both an art and a science, and few people do it really well, either because they don't know you well enough to address the things admissions officers care about, because they can't be bothered, or because they don't know how.
Recommender Tip #1: Academic Recommenders
Many law schools state an express preference for academic recommendations, meaning someone who has taught you in college and can speak to your strengths as a student and a scholar. The reason they do that is because they look to your recommendations to try to predict, as best they can, how you'll do in law school, so they want to get a sense of your talents in the classroom. If the LSAT score is meant to give them an idea of your intellectual horsepower, your recommendations (along with your transcript) are supposed to let them gauge what you do with that horsepower. We all know people who are whip sharp but slackers in the classroom, and people who have to work their buns off to perform well -- admissions officers want to figure out where you fall on that continuum.
If you've been out of college for more than two years, admissions officers understand that it can be very difficult to track down your old college professors, and they'll cut you some slack. If you're in a graduate program, you can ask one of your graduate professors to write a letter. If you're out in the working world, you can ask your boss. If an undergraduate recommendation is at all possible, though, you should try to drum one up, and submit a second, nonacademic one (if required) from one of these alternate sources.
Of your various professors, the most useful ones are going to be those who taught classes that approximate law school the best: classes that are heavy on analytical reasoning, reading, research, and expository writing. Recommendations from classes like Theater, Communications, Creative Writing, Statistics, and Conversational French won't be as useful.
Recommenders that are almost always useless for the purposes of law school admissions include your state senator, friends of the family, relatives, famous people and muckety-muck judges who know you only socially (if at all), your lacrosse coach, and your choir director.
If you're still in school or a recent graduate, and you have some experience working in a legal capacity (as a paralegal, say, or an intern at a legal clinic), you can certainly submit a recommendation from the people you've worked for. Just make sure they are supplemental recommendation letters rather than substitutes for your academic recommendations.
If a school states no preference for the type of recommender they're looking for, assume they prefer an academic one. And if any school gives you instructions that contradict what I'm telling you here, follow those instructions.
Other Recommendation Letter Resources
Recommendation Letter Samples