Your personal statement is your opportunity to give law schools a reason to say "yes" to your file. Don't be afraid to take risks in it. Remember that a mediocre personal statement is wasted paper in your file.
What does a law school look for in a personal statement?
First of all, good writing; anything less than perfection counts as a negative. If you're not sure how good your writing is, look at any of the books I list here.
After good writing, what law schools look for varies somewhat from school to school. Many admissions officers stress that this is your opportunity to address any weaknesses in your file. Some look for leadership, others for community involvement, still others for maturity and seriousness in applying to law school. Many look for "what makes you unique," your diversity in more personal terms; it could be an Olympic medal or your skills as a gourmet cook, nature photographer or quilt-maker. Many schools also look for "personal growth and development," a category which frequently overlaps the one above.
Should I write about why I want to be a lawyer?
No — not unless they specifically ask. Very few admissions officers look for your motivation for studying law. They know that very few applicants have a good reason for choosing it. Those who do ask don't want to hear "I like to argue." They also don't want to hear "I want to save the world" unless there is substantial evidence in your file supporting this: extracurricular activities and internships in the field you mention, for instance.
They do want to hear something thoughtful about how you decided law will enhance your life's goals. As in all other parts of the application, honesty will serve you best.
How do I choose my topic?
I find that the best approach is to first choose something interesting to tell them — why you decided to take up stained-glass making as a hobby, the time you met a bear on a mountain trail, an event that helped teach you humility, independence, or self-confidence. Write a story — 500 words or so — about this topic. Then explain the events in this story by reference to the background you need to show. "Having grown up in West Philadelphia's ghettos, mountains were new experiences to me." "Until I got to college, I had never needed to study; my first year grades showed me I needed to adjust my approach to school." The finished essay should have the story as its central theme, but a lot of other parts of you interwoven.
Finally, a good essay has to have a point — some insight, some moral, etc. "I did X" is meaningless. "What I learned from X" is good. The story should be half the statement, the moral the other half. Stories without points are good for laughs, especially if the story is about body parts or foolish behavior.
But I've never done anything interesting!
I have become so tired of hearing this line, that I wrote a sample personal statement, about nothing at all, to show that an interesting essay is about the writing, not the topic. So herewith I present my most recent scribbling, on k.d. lang.