I hate this stuff. I'm writing my statement of purpose to apply for the JET program where I'd be teaching english in Japan... urgh. I'm supposed to state why I want to go, what I have to offer.
I did a quick Google search and it looks like there are forums and sites which offer somed advice.
"The statement of purpose is really hard because you have to fit this perfect statement in two pages. Mine was about how I enjoyed teaching Japanese college students when I studied in Japan in 1998 and how they really got excited about learning English. I also used it to explain why I failed one class in the study abroad program, food poisoning. Basically say why you would be a good candidate and what you have to offer. Don't mention anything about what JET will do for your future. If you do, at least don't hover on it. It's good to show JET will fit into your life, but you can't make it sound like you need JET for a good job or something." link
Here is a site all about statements of purpose. Mainly geared towards college apps but maybe it will help..