Is there any formal format for statement of purpose? I'm about to submit my application for various PhD programs and I was wondering how should I format my statement of purpose.

Should it be addressed like a formal letter? Just contain my name and say 'Statement of Purpose'? Should I end it with something like 'Sincerely, ...' ?
When I applied to grad school I didn't give my SOP a title or address it to anyone in particular, or include any closing salutation.  Some schools required me to put my user id at the top, which I did.

(I was accepted to several places so I think the way I did it should be fine.)