Call ahead to notify your credit card issuer of you travel plans
Call your card's issuing bank before you travel to let it know that you're taking a trip.
If you do not notify the credit card issuing company of intended foreign travel in advance, you may find your card frozen when you try to use it abroad.
You need to talk to the fraud department and let them know the dates of your trip and the countries you will be visiting. (A few card issuers are now putting a "travel notification" feature on their websites so you can do this in 90 seconds online rather than after five minutes of selecting options in phone call center limbo before you finally get to a live person who can jot down your travel dates and places.)
Why is this?
Most banks have a computerized watchdog that monitors your card's use, looking for radical changes in the frequency or location of charges. When it finds suspicious charges, it freezes the account.
Ideally, this system alerts them if someone steals your card and goes on a shopping spree, but it has the unfortunate side effect of leaving travelers in the lurch, because on a typical vacation you're (a) charging more than usual and (b) charging it from strange places.
(For what to do in case of real theft, see the "Losing Things" section.)