I've rented loads in the US over the last 10 years.
I always use Hertz now.
Book in advance, but don't get a pay-in-advance rate
unless you are 100% sure of your dates as you have no flexibility
for upgrades at the desk and you don't get refunds for early returns.
Sign up to their loyalty card before booking.
Make sure you get unlimited mileage.
gas is cheap so get the biggest car you can afford.
I always go for an SUV if touring National parks
or something like a Mustang, Camero or Corvette for a bit of flash.
Check if your creditcard covers you for insurance.
If it does refuse all
insurance otherwise the creditcard
insurance is invalid and you end up with very limited cover.
Who ever your choose always
ask for deals/upgrades when you get there.
It's the US. There are deals to be made.
Don't rent satnav. Buy one there instead.