The Hidden Fee
Nothing's easier for an ambitious thief than adding on a few dollars worth of "extra" charges when you go to pay at the end of the trip, or when returning a product. Always, always walk through every element of the booking phase when you book, especially with hotels, rental cars, and tour operators. Be sure to talk about cancellation fees, potential late fees, etc., and clear the price up front.

Fake Memberships
We all dream of those alluring travel agent discounts, but be wary of offers to instantly "become a travel agent" by merely purchasing a kit or a simple certification—kits are usually part of a pyramid scam (you "become" an agent by recruiting others just like you), while certificates are typically preceded by a request for "tax" payment. And when you get the certificate, it's really a coupon—and it has expired.

Insanely Cheap Airfare
The ease of online research can quickly lead the wanderlusting traveler to a site with airfare prices that are a fraction of the leading booking sites. You try to book with your credit card, but the agency says it won't go through and advises you to wire the money (sometimes to a major bank)—and hurry, as the price is only good for 72 hours. You jump on the deal. Meanwhile, the same agency uses a bogus credit card to buy the same ticket from a legit site, so that there's a record of the flight on the books just in case you check with the airline. Once the money is wired, the agency closes out the account… and then the real ticket is voided due to the use of the fraudulent credit card.

In short, never trust a booking site that wants you to wire cash.

Membership Numbers
Frequent-flier miles and hotel-club membership points are effectively a commodity in and of themselves—and as such, there have been many instances when someone on the inside (the front-desk clerk at the hotel, a less-than-honest airline employee) will say they've credited your account, but in fact will switch the credit to their own account.

Be sure to double-check that your accounts are properly accredited upon returning home.

The Drowsy Guest
After a long day of sightseeing, you crash early at your hotel, only to get a call in the insanely early a.m. The voice on the other end informs you that this is the front desk and she needs help verifying some information, explaining that some forms were left unfilled and improperly filed, that she just needs to confirm that the information she has is indeed yours. She reads out what she claims are the last four digits of your credit-card number, and asks for confirmation. Upon checking, you say, no, and when prompted you drowsily offer your full credit-card number and expiration date, just to clear up the obvious confusion. "Ah," they reply. "Your form has been found." Satisfied, you hang up and fall back asleep… until you realize (too late) that you just gave out your credit-card number.

Conduct any hotel business face to face—for all you know, the thief is calling from an abandoned front desk.