Know the rulebook so well that you can almost quote it verbatim.
But you gotta know rule 3 very good since that contains probably 70% of the FED-specific rules that differ from pro rules.
If at all possible, buy the case book at
http://www.nfhs.com since that will also help you on the test.
Good luck!
Hint: you can skip rule 9. they never ask anything from it about scorekeeping.