This step is as much for your convenience as for your protection. If you live in Chicago and your credit card company sees charges coming in from Rome, they might conclude someone's already stolen your card and freeze it. This could create a huge headache if you're actually in Rome.
Give the card companies as much detail about your itinerary as possible. Going to Europe? Tell them which countries and which cities, and the dates you'll be there. If you've already made your travel budget, it wouldn't hurt to tell them how much you plan to spend.
Make sure the issuers have a mobile phone number where they can reach you if something goes awry, says Norville. "For a fraud alert, we'll contact you on the number we have on file. Putting your mobile number as one of the numbers we can contact you is a good idea," she says. If you'll be traveling in an area where your cellphone doesn't get voice service, consider buying a locally compatible phone and give the issuers that number.