In Ecuador, which uses the U.S. dollar as the national currency, he could deposit dollar-denominated checks at his local bank, though they generally took three weeks to clear. But Uruguay uses the peso, and local banks don't accept dollar checks.

So, like many retired expats, Harrison operates his finances from the United States. He maintains a Citibank account in the United States and wires blocks of money to Uruguay three times a year at a cost of $45 per transaction. Other retirees also rely on local ATMs to tap their cash in the United States, though fees for currency conversion and non-network ATM use can add up quickly.

Most retirees also keep their credit cards based in the United States. Harrison says he buys lots of merchandise online, "and American vendors generally don't let you use a foreign credit card." Bills also are paid online.

Opening accounts can range from simple to vexing. Harrison's bank in Uruguay "just wanted my passport. It was so easy." For the Millers, the process took weeks. They didn't bring any documents, and the Dominican bank they chose wanted letters of credit and references from the couple's U.S. bank.

Banks are trying to make some of these processes easier. HSBC PLC has revamped its Premier Account to help customers moving overseas arrange for bank accounts and mortgages in their new country. The bank also provides documents necessary for obtaining services such as a mobile phone, which often requires a local credit history.

Charles Schwab & Co. has begun allowing its overseas customers, located in more than 200 countries, to establish standing letters of authorization so they can request with just an e-mail that money be wired to an account abroad.

Health, Social Security

Social Security won't be much of a problem. The Social Security Administration will electronically deposit a monthly Social Security check in many banks around the world, though not all. Still, many expat retirees, to avoid challenges with local banking, have their Social Security checks electronically deposited into their U.S. bank, which they can then access online.

Health care is a bigger concern. Few U.S. employers offer health-care coverage to expat retirees, and U.S. carriers typically don't provide individual coverage to Americans living abroad. Moreover, the federal Medicare program generally doesn't cover costs outside the United States. As such, many retirees either pay out of pocket or, once eligible for Medicare at age 65, return to the United States from time to time for care.