With a few exceptions -- such as a medical emergency as you are passing through Canada on your way to Alaska -- you can't use Medicare in other countries. Still, you should sign up for Medicare Part A, which covers hospital costs, when you turn 65 so you can use it when you come back to the States. It's free, and you can enroll online at www.ssa.gov.
If you don't plan to spend any time in the U.S., you can skip enrolling in Medicare Part B, which covers outpatient services and doctor visits. It costs most new enrollees $110.50 per month (higher-income retirees pay more) and can be used only in this country. Be aware, however, that if you decide to enroll in Medicare Part B later, you'll pay a 10% penalty as part of your monthly premium for every year after age 65 that you delayed enrolling. If you retire before age 65, one option is to live overseas for a while and move back home for good once you become eligible for Medicare. That's what Rosanne Knorr, author of The Grown-Up's Guide to Running Away From Home (Ten Speed Press, $14.95), recommends. But if you're too young for Medicare when you return, you might not be able to purchase health insurance for the first six months after a lengthy foreign stay.
Knorr suggests that you check your current health-insurance policy to see what (if anything) is covered when you're out of the country for extended periods of time. If you take long reconnaissance trips before you move for good, you may want to purchase travel insurance to fill any gaps.
With just a few clicks, InsureMyTrip.com can provide you with several quotes for specific types of insurance, ranging from comprehensive health coverage for single or multiple destinations to emergency medical evacuation. (Without insurance, a medical evacuation could run you tens of thousands of dollars.) Also, try Bupa International, which provides coverage for both travelers and full-time expatriates, and International SOS, which offers help with services such as pre-travel advice, local medical assistance and medical evacuations.
If you don't want to rely on the government-sponsored system of your new country, a number of major insurers offer policies to Americans who travel or live outside their home country. Nationwide, for example, has a plan that will cover you for up to 12 months. American Citizens Abroad offers comprehensive information and health-insurance policies.