Also, provide a self-addressed stamped envelope. A statement of purpose should not be confused with the personal essay, nor is it a prose version of your transcript. Your statement of purpose should prove to the admissions committee your interest in and dedication to the subject of study. Most applicants do not have a well-formed purpose in mind. Do not try to gloss over reality.
Sincerity is discernible and insincerity is unmistakable.
Since a graduate program means extensive work with one or two faculty members, you would be wise to research the faculty in your area of interest. Know who they are, research what they have written, and read what they have written. In your statement of purpose, if you can say you are interested in studying a specific area, explain why and then go on to elaborate your familiarity with the work done by faculty member X in this area. Committee members will see that you have done your homework. Some schools require an interview for acceptance.