I am in the process of preparing for the exam - and I am working with a master casting instructor.
As mentioned, it is not so much about being a perfect caster as about being able to understand how to explain and teach the casts -- and being able to diagnose and correct what the problems are. The instructor I have been working with will do something with the cast and then make me explain what I am seeing and why it is happening.
The advise given to me was to read a lot of the suggested material and watch the recommended videos to get a thorough understanding of the mechanics; practice the required casts; and be able to teach the information required.
Mel Krieger has an excellend video out on casting faults and fixes and every time I watch it I see something I didn't pick up on before -- especially after working with the insutructor.