S.S.
The best way to prepare your child is by feeding him/her a great breakfast on the morning of the test and being upbeat about how they get to show someone all the fun stuff they know!
Really, you can't "prepare" for this test, there are no Cliff Notes, and you may never find out what your child spoke with the tester about.
Seriously, the best way to prepare your child is to read to them every night starting at a young age, encourage the love of books and curiosity in how the world works.
My son is in Kindergarten and we had him tested last year (age 4, in the fall before the fall of Kindergarten) for both classical and gifted. I was told that generally, Classical tests "how much you know" and Gifted tests "how you think". The tests were administered at the same time and in a one-on-one format (teacher/tester and child). So your child also has to be OK with meeting with someone new. It is possible that a child could freak out or be shy and not do well because of fear of interaction with another adult.
We also had our son do the test again this year (since there are a few schools in CPS that only admit in the 1st grade (vs Kindergarten). When the child is in Kindergarten, the fall before 1st grade, they get a different type of test - written - for both the classical and gifted programs (2 tests).
Also, the test rankings are comared to other children at the exact same age, so it is not like you get a benefit from waiting (and getting the test in Feb vs. Sept).
I have been amazed that my child (currently in Kindergarten) could not really read in September of Kindergarten year but by November he was really reading. So for my younger kids, I plan to encourage the nightly reading much more - starting earlier (preschool, aged 3 and 4) and actually challenging them to read to me (which is what the Kindergarten teacher has required us to do this fall - have the child read a book a night to us). This seems to have helped my child jump from enjoying "being read to" and "reading" himself.
Hope this helps.