Do some background research on the programs to which you are applying. You might identify some key professors and read some of their work to give you some background on what type of program it is.
Carefully outline your motivations. Why do you want to attend the program? What to you hope to achieve? What do you plan to do after you have completed the graduate program?
Make a list of the qualifications you have that make you a unique and valuable candidate for the program.
Your statement of purpose should describe your reasons for applying to the program, as well as what unique attributes you might bring to the program. Include a brief description of your background and biographical information, but be sure to tie this information into why you feel you would make a good addition to the program.
You should also briefly acknowledge and provide an explanation for any black marks that appear on your academic record. This allows the admissions committee to gain a better understanding of any blemishes they might see on your transcripts.
You might consider mentioning the names of specific professors that you would like to learn from, or you might mention specific works that have been produced by members of the faculty that have had an impact on your own scholarship. It is a good idea, however, to keep this brief, as you don't want it to appear gratuitous.
You will also want to briefly describe what you plan to do after you have completed the graduate degree.
Once you have completed a draft, revise and proofread it very carefully.