The Snag-A-Cab Scam
Hustlers—particularly in big cities where getting a cab can be a hassle—offer to help you get that ride, telling you to pre-pay them, which they give to the cab driver (with whom they're typically in cahoots). The driver stays quiet and appears informed during this transaction—until you reach your destination, and he asks for more money.

Always clearly establish the cost of the trip before departing.

The Referral Scam, PT 1
You hop a cab and ask to be taken to your hotel, only to be told by the informative cabbie that, alas, the hotel in question has been closed/burned down/raided by narcos. But they just happen to know of a great hotel—one where they get a commission for every customer they bring through the front door. Or, even worse, they agree to take you to your hotel, but actually take you to their hotel instead, insisting it's much better.

Call and confirm with your hotel upon disembarking, and ask if they can arrange for transportation—and when you pre-arrange, confirm the price with the hotel and the taxi driver before departing. Even if they don't offer transport, they will give you a ballpark rate with which to work.

The Referral Scam, PT 2
You ask to go where you want to go, and then ask, "How much?". This typical bartering strategy in developing countries is then met by an insanely low price, provided you're willing to make just one stop. That stop, of course, is at a souvenir shop, shady gem merchant, or a sub-par tailor, a place typically well out of your way where the salesmen will be aggressive and the deals too good to be true. Leave empty-handed, and you'll be expected to pay more than the agreed-upon price when you arrive at your destination. To sometimes sweeten the deal of that "one stop", you'll typically hear that your end destination isn't open yet, but your ever-so helpful guide can help you kill that extra hour you didn't know you had.

If it's too good to be true, it's usually because it is.

The Airport Express Lane
When prepping to fly out of an international airport—or sometimes when disembarking in an international airport after customs—a friendly fellow buddies up and says he's part of an "express luggage" service for airport check-in, or will help transfer your bags to… wherever. Only it's not a free service, and money will be demanded.

Only accept assistance from uniformed airline/airport employees.