Graduate study is not for slackers. It takes focus and determination to pursue an advanced degree. That's why admissions committees examine your statement of purpose very closely—they want to see whether you have the right stuff to succeed in graduate school.
Different schools have different prompts. Nonetheless, they're all asking for the same four pieces of information:
Admissions committees look for candidates with clear, well-defined research interests that arise from experience. With that in mind, your statement should reveal that you care deeply about your chosen discipline and that you have the background to support your ideas and sentiments. It should also demonstrate that you're a diligent student who will remain committed for the long haul. Always answer the question asked of you. Being substantive and direct is much better than being creative or flashy.
Grad schools don’t care that you make a great chicken casserole or play intramural bocce ball. They do care about those activities that speak to your suitability for graduate work. As a graduate student, you'll be called upon to do difficult coursework and research. You may have to teach undergraduate classes within your field and conceivably even design a course. And you'll have to get along with a diverse group of colleagues who will sometimes work very closely with you. Any experience in school, work, or your extracurricular life that speaks to those abilities is worth talking about.
While it's important to be focused, there's no need to be boring. To distinguish your essay, add unique (yet relevant) information. One of the best ways to do this is to discuss—briefly—an idea in your field that turns you on intellectually. It's an effective essay-opener, and it lets you write about something besides yourself for a bit.
Remember, the idea you choose to talk about can tell an admissions committee a lot about you. And it demonstrates your interest in your field, rather than just describing it.
Be sure to show your statement of purpose to someone you respect, preferably the professors who are writing your recommendations, and get some feedback on the content before you send it in. Have someone else proofread your essay for spelling and grammar. A fresh set of eyes often picks up something you missed.
Finally, don't just reuse the same statement of purpose for each school to which you apply. You can recycle the same information, but make sure you change the presentation to fit each school's individual program.