I did a 4000 mile trip with little to no preparation this August, infact I basically just drove West from Chicago without even a plan only knowing I was going to San Francisco eventually.
Driving itself was a piece of cake bearing in mind the car was automatic and had cruise control along with A/C but I would definitely recommend making sure you have at least half a dozen bottles of water at any given time, it was ridiculously hot in some places (pushing 48C in Vegas when I was there). Some power bars and little necessities like wet wipes, hand sanitiser wouldn't go amiss as some of the road side toilets were damn right terrifying!
Another thing is make sure you have some good music/ipod with you, something I overlooked and as a result every time I will forever associate Taylor Swift's 'We Are Never Ever Getting Back Together' with my road trip as that song was released just about the time I did the trip

Most places like Nebraska, Arizona, Utah and Nevada played nothing but freaking country and that's if I was lucky, some places I wouldn't get any radio stations!
Good camera is a must as you will see some amazing sights, infact I bought HD Hero 2 just for the road trip and strapped it to the dash for some time lapse goodness, have gigabytes of photos I still need to assemble into some sort of a video.
Lastly though do stay vigilant and alert when driving, it's easy to become little complacent when you cruising around highways with little to no traffic and particularly driving at night when something springs on you suddenly. Infact I had rotten luck in my last 2 days of driving, first someone blew a tyre which then bounced off a truck and hit me just as I was overtaking said truck, didn't even have time to react as this was in pitch black darkness. Tore the bumper at the front and damaged the car underneath as I drove over it, couldn've been far worse bearing in mind it happened at 70mph. Got a new rental car and the following night a coyote runs right in front of me, I braked and swerved but still hit it hard and another bumper needs replacing along with you know, killing an animal.
Slightly freaky thing is that both of those happened around the same time and bumpers were both hit in the same place but the morale of the story is buy insurance! Not a question was asked, just filled an incident form for both cars and nothing extra to pay. Oh and don't drive in a middle of nowhere in a middle of the night like I did!
EDIT: Oh and one thing I realised after I finished this is that there wasn't a single speed camera on the way but the police over there can be vigilant with their speed guns, saw a lot of them pulling people over in Nevada!