It helped that I had many years of cybersecurity experience. I had already passed the Certified Ethical Hacker exam. I read the CISSP CBK. I also read several books on encryption, since that seemed to be a weak spot. Specifically Crypto by Steven Levy, and The Code Book by Simon Singh. A 40 hour boot camp. Plus participation in a CISSP Study Group put on by our local (ISC)2 chapter with the local ISSA chapter and the State of Minnesota IT department (MN-IT). And I had a practice exam that was helpful, but nothing at all like the actual exam.
Took the test, was pretty sure I was failing, but passed it on my first go.
The training I had already for the CEH and the career experience were probably the most important factors that contributed to my success. Can't really beat experience, which is why you need 5 years of experience to complete your certification.
Security+ might be an easier way to get some success in testing before trying the CISSP.