Name + LSAC Number in header, on the right
No creative titles, just Personal Statement or Diversity Statement
PS or DS as the title, not in the header as well?
are there standard guidelines somewhere online?
I totally understand the paranoia of making sure everything looks right for something so important, but at the same time, you're applying to grad school, and so I think most of them are going to assume you can format a paper in a presentable way. There aren't "standard guidelines;" just read the directions. Some schools will tell you to label the statement with the question number from the application, but most just tell you to go for it and give you a suggested length. Make it presentable, clear-cut, organized, and you're set.
That said, in my headers, I put my name top left, the title (which most of the time was ____(school name) Personal Statement), and "LSAC #______" top right.