I took a 1 week class offered by "TrainACE" in Maryland. I had a excellent instructor and we used the "CISSP All-in-One Exam Guide" by Shon Harris. After the class, I purchased another study guide and reviewed that. Then when I felt ready to take the exam, I started taking practice exams until I consistently scored in the mid-80's on them. I also agree with the scheduling the exam to give yourself a hard date to prepare for. I know it helped to force me to buckle down and make sure I was ready.
Even after all that, when I walked out of the exam, I was exhausted!
Back when I tested, there were a couple of websites that had free practice exams. I read the official CBK to a large extent, then would take practice exams to help identify week areas, then I'd go back to the CBK and spend more time in the areas I was week on.
I know the site that I used no longer has practice exams for CISSP. If you have taken any training classes, they probably provided some test prep questions, so I'd review those to start. Likewise, any area you know you are weak on, buy the CBK and study those areas specifically.