It's quite easy for cabbies to swindle money out of an unsuspecting tourist. They prey off your being a stranger in a strange land.
Again, it's important to be knowledgeable about the local currency. If you hand a cabbie a large bill, he may drop it and then pick up a hidden smaller bill, insisting you didn't pay enough. Some will charge higher nights and weekend rates on the meter during a weekday. Some might quote you one fare and charge you double once you arrive at your destination.
Some cabbies work in tandem with local hotels, offering you a "package deal" right at the airport. However, once you arrive at the hotel, you find the discounted rooms mysteriously all filled up. The hotel then charges you an exorbitant amount to stay, and the cabbie receives a commission on the side.
It also pays to have a working knowledge of where you're headed. If you know the route to your hotel should only be 15 minutes, make sure the cabbie knows you know this. Don't let yourself be talked into alternate routes or "shortcuts" that could end up lasting 45 minutes or more.
How do you avoid falling victim? Make your travel and lodging plans in advance. If you need a taxi, use a hotel or restaurant to call for one. If hailing a cab on the street, choose only those with prominent logos and telephone numbers. Never accept a ride from an unmarked car -- you could end up losing more than just your wallet.