No matter how long or short your trip, make sure you have adequate travel insurance, purchased before departure. At a minimum, you need coverage for medical emergencies and treatment, including hospital stays and an emergency flight home if necessary. Medical treatment in the USA is of the highest caliber, but the expense could bankrupt you.
You should also consider getting coverage for luggage theft or loss and trip cancellation. If you already have a home-owner's or renter's policy, see what it will cover and consider getting supplemental insurance to cover the rest. If you have prepaid a large portion of your trip, cancellation insurance is a worthwhile expense. A comprehensive travel-insurance policy that covers all these things can cost up to 10% of the total outlay of your trip.
If you will be driving, it's essential that you have liability insurance. Car-rental agencies offer insurance that covers damage to the rental vehicle and separate liability insurance, which covers damage to people and other vehicles.
Worldwide travel insurance is available at www.lonelyplanet.com/travel-insurance. You can buy, extend and claim online anytime – even if you're already on the road.