Writing a statement of purpose can be frustrating. There isn't a lot of information to help streamline the process. Follow these simple guidelines and you'll create a great statement.
Keep in mind that that all Statements of Purpose (SoP) ask for the same four general things: what you want to study at grad school, why you want to study that topic, what experience you have in that field, and your future plans to use the degree you'll be attaining at their university. All tailored to their university, of course.
Make sure to read the school's requirements for the statement of purpose. Each one is unique and will ask for a different emphasis.
Be specific. It's important to the school to know why you chose them over others. This isn't undergraduate studies where you could be general. For any school you apply to there should be a hook that interests you about that specific program and why it's tailored for you and your future. A great example is the types of research/emphasis areas they teach.
Don't simply rehash the information they can see in your other submitted materials. Your SoP is your best chance to show off your individuality. At the same time, remember this is a professional advancement, and should be treated as such. Don't write embarrassing or private information unless it's to make a very good point.
Adhere to any guidelines they outline for the SoP. They're there for a reason, and a sloppy SoP will get your application tossed out.
Use good grammar and punctuation. I work in a graduate program, and you would be surprised how many people don't use capitalization and punctuation. It's one of the top things admissions coordinators complain about, it makes you look unprofessional, and it will count against you if you can't even place a period at the end of a sentence.
Make sure to have someone check it over once you're done. Professors who are writing your recommendations are best, followed by anyone with writing experience who can catch any obvious flaws in your statement and improve it.
If you can write a tailormade statement of purpose for grad school, you'll impress the review committee and give yourself a leg up in the process. Good luck!