Let me share my extensive wisdom from my eleven weeks of experience...
-- Make sure to spend some time in the mechanics manual...
-- Establish your pre-snap checklists for each position that you'll be working... And practice them...
-- If there are rules classes in your area, attend them...
-- Same goes for Officiating Camps...
-- See if you can get on the sideline of a varsity game or two... (Work chains, track penalties, etc...) I think you can learn more watching a game up close when you don't have to worry about what you're supposed to be doing...
-- Learn your signals... (I recommend making some flashcards...) Even if you're not gonna referee, your white hat may forget a signal, and it'll be useful if you can help him out...