The statement of purpose is your opportunity to discuss your research interest and to persuade the admissions committee that you are intelligent, knowledgeable and that your interest and skills make you the best candidate for their program you are interested. Therefore, your writing should be clear, well written, creative and organized. The key to a strong statement is asking others for feedback and revising, revising, revising!!!
Here are some resources to help you write your statement.
What Do Colleges Look for in a Statement of Purpose?
This assay examines in detail what colleges are looking for in a stamen of purpose.
Pathways to Science
Their resource library tool contains information on writing strong essays and statement
Writing a Winning Statement of Purpose
Two PowerPoint presentations from UCLA show the steps to writing a winning statement of purpose.