Although not dissimilar from the resume you'd prepare for a job search, your law school application resume should highlight the experiences you've had that are most relevant to success in law school and the legal profession.
Specifically, focus on:
your reading, writing, and analytical skills
If your college major is in an area where you wouldn't generally be expected to do a lot of writing, it's particularly critical to quell any doubts about your ability to handle the close reading and writing workload in law school. Including examples of lengthy, non-technical papers you've written can help, as can highlighting coursework that included a heavy reading load.
A Pretty Resume is an Effective Resume
Keep in mind that your resume is an indication of your writing ability. It goes without saying that it needs to be technically perfect. Misspelling and grammatical errors must be avoided.
Also think about how it looks. Is the alignment perfect? Have you used bold, underlining, or italics consistently?
Your goal should be to have a resume that pops when someone glances at it. Make the most important information leap off the page, using the layout and fonts to highlight the most important elements.
One useful technique for seeing what the reader is going to notice first is to print the resume and tape it to a wall across the room. Start far enough back that you can't read anything and slowly walk forward until the writing becomes legible. What do you see first? Is that what you want the admissions committee to focus on? If not, revise the formatting until the most important elements are highlighted.
A Concise Resume is an Effective Resume
Unless the school requests otherwise, it's not necessary to include every single job you've ever had. Feel free to drop your summer lifeguard job when you were sixteen. That being said, including some fairly menial jobs isn't the worst idea, as it shows that you've had a good work ethic from an early age and demonstrates that you're able to balance competing demands on your time. Just don't waste a ton of space detailing your responsibilities for these early jobs. Focus on what's most important.
In case you're considering embellishing your background, don't. It's bad karma, and it might come back to bite you if the bar examiners find out you lied on your law school application and refuse to admit you to practice.
One Surefire Way to Improve Your Resume
When you think your resume is finished, print it out and go through it line-by-line to make sure it's perfect. Then give it to at least two people you trust, and ask how you could improve it. Repeat this cycle until you're all convinced it's as persuasive as it can possibly be.
Then wait 24 hours, read it again, and make sure there's nothing else that should be changed. At that point, you're finished!