Make sure you've completed the syllabus your flying school has in their ops manual, with nothing left out. If your instructor has graded anything as not to standard, make sure it is fixed and documented before the test. They are very particular about making sure the course has actually been completed. They are also looking to see some direct involvement of the CFI in your training, so at least get him/her to listen to some briefs and go for a fly with you.
CASA have said the majority of people being tested are passing, however a good deal of people are not actually attempting the test because in the eyes of the examiner, they haven't completed the course.