Elon University's pre-law adviser, Dr. Nim Batchelor, suggests on LSAT Blog that students should begin thinking about their personal statement for law school by imagining that they have to describe their "real self" to the admission committee. He suggests thinking back through your life and finding 10 or 20 stories or episodes that you might use to tell others about your life, then selecting four or five to use in your essay that demonstrate who you are, show an important part of your personality or illustrate something you are proud of. Your essay should get across the idea that becoming a lawyer is a "natural extension" of your life story.
Your personal statement should illustrate your character and personality and demonstrate that attending law school is the next logical step in your life. You should demonstrate through your essay that you have a strong and well-thought-out commitment to law school. Your writing should be flawless and should not have any grammatical errors. When describing events that shaped your life, include discussion of how you reacted to the events, and how this has influenced your outlook and desire to attend law school. Try to focus on something that is unique to you, and is not likely to be repeated in other essays.
The University of Chicago Law School recommends that you try to avoid simply rewriting your resume in your personal statement. They also caution against putting anything too odd or gimmicky in your resume just to stand out from the crowd. You may end up standing out for the wrong reasons. If you are going to include any legal arguments or information in the essay, make certain it is correct -- some or all of the members of the admission committee will be lawyers. Avoid making statements about yourself, such as "I am very interested in helping others," without also including a personal story that demonstrates and backs up your statement.
The Student Academic Success Center at the University of California at Davis advises law applicants to avoid making excuses for problem areas such as breaks in your education or a poor exam result. Always use clear language and try not to use legal jargon or overly complicated phrasing. You should write in the first person and avoid passive verbs and phrases. Proofread your work and have a friend, or several friends, read over your essay. Keep in mind that you may go through several drafts, and leave yourself enough time to do rewrites. Many universities offer workshops on writing a law school personal statement for current undergraduates -- check with your university's pre-law adviser.