Bring lots of money!!! A friend of mine told me it was $20 per day for long-term parking in the parking decks - if you wanted the open economy lots off in the far corners (in my opinion, not nearly as safe - especially in Newark, the car-theft capital of the country).
In the end, for our 8 day trip last month, it was actually cheaper for us to get a cab round-trip than to park at the airport.