United States hams operating outside of US... what to do?

Operating Overseas Operating Overseas FAQ
1) Does the country you will be visiting participate in a multilateral reciprocal operating
authority--CEPT or IARP? If so, operate under CEPT or IARP.
2) If it does not, can I obtain a Reciprocal Operating Permit even if the country does not share a Reciprocal Operating Agreement (bilateral) with the US? Yes
3) Are you traveling to Canada? The US and Canada share an automatic reciprocal operating agreement. How can I operate outside the US?

You can find a complete listing of the requirement for a country or countries at Operating Information by Country. This includes most countries, including CEPT and IARP participating countries.

How can I operate when CEPT or IARP isn't possible?
Yes. It is possible to obtain a permit a permit for almost every country in the world. Although ARRL maintains paper files at HQ, the most up-to-date information on obtaining permission to operate in a country can be found online at the ARRL Web site or on the Web site of Veikko Komppa, OH2MCN. ARRL HQ and Veke, OH2MCN, work together to make sure that up-to-date information appears.

This can include information on the national Amateur Radio society, repeaters and local clubs. Information on travel warnings in a particular country can be obtained from the US Department of State with the primary purpose of alerting the public to adverse conditions in specific countries.

Are there guidelines for obtaining a permit? The most complete information appears on the ARRL Web page. If specific application information for a country on this page is unavailable or unclear, write a letter of request or send an e-mail to the countries telecommunications authority for a permit. Include information on the purpose of your trip, the dates and place(s) of your stay, your passport and the equipment you intend to use. Attach to it a photocopy of your amateur radio license issued by FCC. In some cases where Amateur Radio is not widespread, a letter attesting to your character signed by the chief of police (or equivalent) of your hometown might help if attached.

Submit your application as much in advance of your trip as possible. It may take 30 to 90 days or more to be processed. Do not forget to keep a photocopy of everything you send for future reference. This does not guarantee that you will get operating permission, but it is a start. In many cases, it is important to have contacts in a country and the IARU society of that country may be helpful. What are my privileges are in the country I will visit? When operating under CEPT or IARP, there are two classes: Class 1 licensees are those who have demonstrated proficiency in Morse code to the licensing agency. They may operate with the same privileges they are authorized in their home country provided that they do not exceed those privileges granted to the highest class license available in the country. Class 2 licensees have not demonstrated proficiency in Morse code to their national telecommunications agency and are limited to privileges above 50 MHz. If the country does not participate in CEPT or IARP, the privileges are whatever the telecommunications agency granting the reciprocal operating authority says that they are. If not specified, the ITU Regional provisions apply generally, but there may be exceptions. How can I operate my station in Canada? When a US amateur operates in Canada, simply bring your FCC license, proof of your US citizenship (a birth certificate or other proof) and identify as call / Canadian identifier, like N1KB/VE3. At least once during the communication, you must state your geographical location, like "30 km north of Toronto."

http://www.arrl.org/us-amateurs-operating-overseas