Pretty much nothing to say that hasn't been said so far.
A XM radio is must.
Wax the front of your car/truck with good quality wax. Twice.
Call ahead and make yourself reservations at hotels for the night before you start your trip. Nothing worse than stopping for the night to learn the state WOW convention is there and there isn't a vacancy for 100 miles.
A squeegee to scrub bugs and junk off your windshield if the gas station doesn't have one. And a jug of washer fluid.
Baby wipes and a change of clothes.
Motrin. You will be surprised just how bad you ache after 15 hours in a car.
Imodium. Never know when the shits will hit.
And if your in a hurry, plan your drive to be mostly at night. Very little traffic.
550 cord. This stuff saved me on a trip from Ft. Hood Tx to Ga when a recap got wedged in my rear leaf springs. Tied it around the front tire and the recap and snatched it right out.
Sunglasses. And not the cheap ones.
Ga to Me and back several times.