The first paragraph of your statement is important. It should be memorable and say something real about you and your career choice without being trite or formulaic. If applying to graduate school to pursue a career in astronomy, for example, do not write "I'm applying to this program because I love astronomy and want to study here."
Instead, make it memorable by saying something such as" My dad and I watched the Leonid meteor shower every year. It created a desire in me to know more about the stars and I read everything I could find on astronomy. That first telescope opened up a whole new world to me. I want to make studying that world my life's work."
The Graduate Opportunity Program at U.C. Berkeley advises prospective students to think of the statement of purpose as a composition with three parts. The opening section should describe the program that you are applying to and the area of research you wish to pursue.
The second section should detail the work that you have done in college that prepared you for graduate school. It is important that the readers get a sense of your dedication to the subject and that you can handle the graduate studies work well.
The third section should identify who you want to study with. Mention professors that influenced you and any papers or lab work you took part in. If there is a professor at the graduate school whose work truly interests you, indicate why you would like to work with him. Show a real familiarity with this teacher's work and publications.
As in any composition, the first draft of your statement of purpose is going to be bad, wrong or both. Make sure that your paper is grammatically correct with no typos or spelling errors. After you do the revision, ask one or more of your professors to read and critique it. He can give you some insight as to what the professors reviewing this part of your application are looking for.
Rewrite it as many times as you need to get it right. According to Gordon Haas of the University of Alaska, you want to demonstrate clearly what your graduate school and career goals are and that your previous experience and education have left you well prepared to undertake graduate studies.