The SoP is the part of the application which likely plays a huge part in any success. Take your time crafting the best SoP you can. Your SoP should elaborate on the strengths you've brought up in your application, and should refer to the traits JET is looking for. Provide brief anecdotes about experiences you've listed in the application. Show some personality, give the interviewers a reason to remember you. Also, have other people look over your SoP. This is a professional-caliber essay, not something for an intro English class.

The Top Ten DO NOTS:

1. Detailed discussion of mental or physical health issues.
2. Serious grammar, zero punctuation and/or spelling mistakes.
3. Not answering the question and/or very very very long sentences that never actually reach a point
4. Criticising anyone or anything.
5. Writing too much or too little.
6. Talking about what you want, instead of what you can do.
7. Giving examples, especially long winded ones.
8. The mention of anime, manga, or video games.
9. Simplistic interests. If you have a personal interest in an aspect of traditional Japan, mention it, but tread cautiously. For example, it's fine to enjoy ikebana or karate, but don't spend the majority of your essay talking about it.
10.Make sweeping statements about Japan/Japanese which may be insulting and/or patronising

The personal statement should be between 800 and 1000 words long, word processed, font size 12 and double spaced on A4 paper. As long as your statement fulfils the above criteria, it will be acceptable (disregard the maximum page limit on the application form). Further to Point 2, it should be devoid of spelling and grammar errors. It should flow well. It should sound good when you read it. You do not want to be remembered as the candidate who couldn't tell the difference between "its" and "it's". Remember, you're applying to teach English to Japanese students. It won't reflect well on you if you don't have adequate command of the English language yourself. Have other people review and edit your essay, preferably trained professionals (like English or Journalism professors). If you don't have professionals, use who you can find. Just make sure that you have this essay as polished as you can make it before you submit.

There are three broad reasons why I would like to participate in the JET program which also encompass my career goals. The first consideration is that all of my university studies were structured around internationalist issues with the aim of eventually working overseas, more specifically within the Pacific Rim area. My studies in California were also undertaken with this goal in mind, along with my travels throughout the Pacific. I would particularly like to teach and have applied to undertake a graduate diploma in Education with TESOL as a my major. However I consider the JET program offers a much better window of opportunity in this regard...

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My interest in Japan and Japanese language developed when I lived in Switzerland during middle school, where I had a Japanese friend who taught me a few Japanese words (cold, warm, etc.). When my family moved back to the US my parents chose where we lived based on whether or not the local high school offered Japanese. I am very interested in foreign languages, but after four years of studying Japanese it has become my favorite, and my current goal is to become a Japanese-English translator / interpreter...

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My childhood was what one would expect of an American kid. I grew up playing with my sisters and my dog in our backyard. My friends were “typical” like me- white, middle-class, and Christian. I was happy but, being the middle child rebel that I was, I constantly wanted to do something to be different. Therefore, while my sisters and friends played soccer every summer, I took up karate. Over the years, I became close with my karate instructor and his family. He often talked about the years that he and his wife spent in Japan living with his mother’s (a native of Japan) family after college. Eventually, I decided that I too would try to spend some time in Japan- learning the language, exploring the culture, and doing something different and exciting...

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If I am successful with my application as an Assistant Language Teacher through JET, I will spend the spare time I have studying Japanese after preparing for my classes in school and at home. I will resume my studies of the Joyo kanji, kanji compounds, sentence structures and spoken Japanese.

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Language. It is a sign of who we are and where we come from. As language defines us, so does it unite us, but it can also impose barriers that drive us apart. As our society aggressively pursues globalization, individuals who maintain cultural sensitivity and strive for effective communication despite language barriers will be an increasingly important commodity; individuals who can also pass the gift of adept communication to others will be invaluable...

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I have always done a lot of traveling; by the time I was twenty-one I had stepped foot on four continents. Growing up, I spent school years in Massachusetts with my mother and summers in the Western Pacific with my father. I also lived in England for a year, and in the last eighteen months I have visited England, Costa Rica, Mexico, Grand Cayman, and China...

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Out of all the countries I have ever visited, studied, or expressed interest in, Japan has always been the one in country Asia I wanted to travel to most of all. I was always interested in different cultures and countries due to my parents and their friends who loved to travel around the world. Exploring how different groups of people lived and trying to understand why they were different from my own culture were a hobby of mine. In fact, it’s quite surprising I didn’t become a geography or sociology major in college. Asia was probably my favorite continent; I loved to read about the various countries, they’re histories, classic books, design, and ways of thinking. However, my initial knowledge of Japan and why it’s my favorite country in Asia came from my neighbor Hiro who originally lived Sapporo before moving to the USA...

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The Japanese language and culture is the foundation from which my interest in Japan and the Jet Program has grown. I have always been interested in teaching English as a foreign language. The idea of improving the cultural awareness of students both in Japan and in my country and giving students a reason to speak useful English has been a dream of mine for many years. I understand most of the students have a low level of interest and motivation in learning English and my goal is to stimulate their interest by working with someone from the world outside of Japan with a different background and culture...

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Other useful discussion on the statement of purpose is here and also here

An outline from JET-UK is here