To obtain one, go down to your local AAA with two passport photos (with your signature on the back of each) and a copy of your state-issued license. You will be asked to pay a fee of $10 for a one-year issue and fill out an application, which can be printed online beforehand if you want to get a head start. If you can't get to a local office or would rather apply by mail, print and complete the application and send it with two signed passport photos, your $10 payment and a photocopy of both sides of your driver's license to:

AAA/IDP
1000 AAA Drive
Heathrow, FL 32746
Attn: Mailstop #28

Beware of phony IDP's! Only two agencies in the U.S. are authorized to issue IDP's: the American Automobile Association and the American Automobile Touring Alliance. Hundreds of Web sites sell fakes, many charging over $100, believe it or not. For more information or to locate the AAA nearest you, go to the official AAA Web site.

Be sure to get your IDP before leaving home, as it must be issued in your home country. An IDP is not a license itself, merely a translation of the license issued in your country of residence. If you are stopped by law enforcement officials abroad, you must present both your IDP and your U.S. license. The only driving record you have, therefore, is within the United States, so obey the local driving rules! Any infraction or citation issued abroad will be waiting for you when you return home.

Check with the consulate or embassy of the country you're visiting to find out their policies on international drivers.

Source: Independent Traveler.com