Pack light if you can as you don’t want to be carrying a heavy suitcase every day. If you’re travelling during the summer months bear in mind that it can get extremely hot and so pack accordingly - shorts and tee-shirts will generally be fine!
If you’re renting a vehicle it’s almost a given that you’ll have air conditioning but even so it makes sense to always carry several bottles of water with you, particularly when driving through the more remote desert sections. I bought a cheap crate of bottled water (I think over 30 bottles!) from a supermarket and left them in the boot, occasionally restocking the glove box.
If you’re new to the States and fancy having a beer to finish off a day’s driving be sure to carry photo ID with you as you’ll always need to show ID to get served in bars. Obviously, if you’re a foreigner you’ll have your passport with you anyway - just remember to take it out with you in the evenings! Also remember to take your driving license as - aside from needing it to hire a car - in most states it’s against the law not to carry it with you.
A specialist Route 66 map is essential. A highly recommended map and guide is “Route 66: EZ66 Guide For Travellers” by Jerry McClanahan.
Aside from that just remember to take a camera to capture those memories!
