There are common scams that occur in many places that the traveller should be aware of. These are designed to get your money or business from you under false pretenses. They fall into three categories: overcharging you, deceiving you or coercing you into paying for a service you don't want, and outright theft.
Prevention is based on knowledge: researching your destination will both alert you in advance to scams in the area and let you know what the usual prices and truly good sights are, so you will be less reliant on the approaches of helpful individuals when you're vulnerable.
At the same time, if you do get stung, don't be too hard on yourself: you were dealing with people who knew the location a lot better than you and with people who were out to deceive you. In some cases you were dealing with hardened criminals. If you think what happened to you was illegal and the police are trustworthy, report it, otherwise you'll have to chalk it up to experience.
These scams are based upon the idea of offering you help or advice that is actually deceptive, trusting that you will rely on the scammer's "local knowledge". They usually involve giving advice that results in you paying for something you otherwise wouldn't or going somewhere you don't want to go. Some scams in which a helpful local offers to cut you a good deal can be outright fraudulent — convincing you to buy fake gems for example — but many simply get you to pay for something that you wouldn't pay for if you knew the area better.
One of the biggest traps of these kinds of scams is the desire to be polite to people who are polite and friendly to you; and the scammers know this. While you shouldn't become a hard-nosed nasty person, you should receive unsolicited offers of help with polite caution, and when you are reasonably certain that you're being scammed, there's no need to be polite in fending it off: feel free to walk away, yell at the person or yell for help.
Another trap is the "too good to be true" offers: they are almost certainly not true.
Your driver or guide will tell you that the place you're heading to is gone, or if you've booked, that it's no good or too expensive and that he knows somewhere better. While this may be true, it's likely that the 'better' place is giving him a commission for referrals. You could either insist on going to your planned destination or get him to agree that if you don't like his recommendation then he will take you to your original destination. The latter will often work if his commission only depends on getting you to the door, but it's hard to tell what the deal is. If you're worried, just insist on your original destination. In some cases the driver will not drive you to your hotel even if you insist. Sitting with your luggage on the back seat and threatening to get out at a red light without paying usually helps.
You may arrive at a major tourist destination only to find a very helpful local near the entrance explaining that there's a riot/holiday/official visit at the place you want to go and it is closed. The local will then offer to take you to a lesser known but infinitely more beautiful sight or to a nice shop. Generally the destination is in fact open for business: refuse the offer and go and have a look. Even on the rare occasions when they are telling the truth they may not be as helpful as they seem, so it would be better to pursue your own backup plan.
These scams are based on your ignorance of the area and rely on getting you to pay well over the market rate for goods or services. Some will rely on a helpful local steering you to the goods, but others will simply involve quoting a high price to you. In some countries this is institutionalised: foreigners have to pay more even for genuine sights.
Getting a general sense of accommodation price ranges and the like is the best way to prevent being overcharged. In some places it's assumed that you'll bargain down overcharged prices, in others you will just have to walk away or pay up for goods although you should still challenge the amount in the case of a service if it is clearly overpriced.
Scenic taxi rides
Since you don't know the area, taxi drivers can take advantage of you by taking a long route to your hotel and getting a large metered fare. The best prevention is knowledge: it's hard to learn a new city well enough to know a good route before you arrive for the first time, but it might be a good idea to ask your hotel roughly what the taxi fare should be when you book or to arrange a pickup with them if they offer the service. Often you can negotiate a fixed price with a taxi before you get in.
Gem and other resale scams
You are taken to a jewelry shop and offered a once-in-a-lifetime opportunity to purchase gemstones or jewels at special discount prices. Another customer in the shop, well-dressed and perhaps from the same country as you, tells how he made incredible profits last year by reselling the gems and is now back for more. But hurry! The sale ends today and you have to pay cash.
Of course, once you get back home and try to sell your booty, it turns out to be low grade and worth only a fraction of what you paid for it. This scam is particularly prevalent in Bangkok, but variations on the theme with other products that can supposedly be resold for vast profits are common elsewhere too.
These scams rely on trapping you in a bad situation and forcing you to pay money to get out of it. They're best prevented by avoiding the situation; once you're in it you may well have no option but to pay whatever it takes to get out of it safely. Many of these scams are bordering on illegal.
You are offered a "free tour" of a shop or factory way out of town. Your driver may then suggest that you'll need to buy something if you want a ride back. The best prevention is avoidance, as if you're stuck out there you might well be compelled to do as she 'suggests'. Don't accept any kind of lift or offer of a tour without having a basic idea of where you're going and whether you will be able to get back if your driver deserts you.
You are met in the street by people who say they are art students. They speak English well and invite you to visit their school. Then they will try to get you to buy one of their works for an excessive price.
A few bars in China, especially Shanghai, will give you a menu with reasonable prices to order drinks. Later they present a bill with much higher prices. If you argue, they produce a menu with those higher prices on it.
The best way to avoid this is to stay out of sleazy tourist bars. You could also try hanging on to your menu or paying when your drinks are delivered, preferably with the right change.
Passport as security for debt or rental
You rent equipment like a jet ski or motorbike. You are asked to give your passport as a security guarantee. After returning the rented goods, the owner claims you damaged them and will ask for exagerated prices to compensate. If you do not agree, they threaten to keep your passport. People also lose passports when holding them as security or guarantee for payment of a debt. This scam is used in almost all Thai tourist resorts, and is very effective.
Never hand over your passport as a security or guarantee in any circumstances.
These scams are outright theft: they involve putting you in a position where someone can take your money by force.
See also Pickpockets.
Credit card skimming
In this scam, you use your card to pay in a bar or restaurant. However, while your card is out of your sight, it is swiped not only in the machine that sends the information to your bank for approval, but in a second machine which copies the card's identifying information from the magnetic strip. The copy of the card, or the number, are then used by the third party to buy goods. Often this is an "inside" job: employees of the outlet are either using the information themselves or being paid to acquire it.
The best way to prevent this scam is to keep your card in your sight at all times. Unfortunately the typical restaurant custom is to let the restaurant staff take your card away and bring you back a receipt to sign: insisting on observing them while they handle your card may make you unpopular.
Otherwise, you can limit the damage done by credit card skimming by keeping receipts when you use your card and checking them against your credit card statement. Make sure the amounts match up and make sure there are no additional purchases you didn't make. Report any discrepancies to your credit card company: the liability rests with them not with you, as long as you report fraudulent transactions as soon as possible.
The Maradona is a scam that is very common in Romania, especially in the capital Bucharest. Someone will approach you and attempt to engage you in a conversation (in English), typically - although not always - about something vaguely illicit. Seconds later, two men will appear in plain clothes but flashing legitimate-looking police badges. They will accuse you and your "new acquaintance" of some illegal activity (usually 'currency swapping', a totally ridiculous charge in a country where legitimate currency exchanges are more common than streetlights), and demand to see your wallet and/or passport.
Do not hand them these things! Keep your documents and belongings in your pocket and out of sight....
Walk away, or yell, or tell them outright that you do not believe that they are the police, or suggest that you all walk to the lobby of a nearby hotel (or police station) because you are not comfortable taking out your wallet or papers in the street, or whatever. These con men thrive because the police fail to enforce laws against non-violent crime and because some foreigners are easily gulled. They will not physically attack you: the treatment of violent offenders is severe - these men are professionals, and they would never be foolish enough to chance a physical attack.
Distraction thefts take a variety of forms. Generally the thieves work in groups: one will distract you and the other will rob you while you're distracted. Sometimes a single thief will rely on a ready-made distracton like a busker or a departure board. Sometimes the distraction can be pleasant, such as having an attractive accomplice talk to you, but sometimes it's very nasty, such as throwing rotten eggs or faeces over you and robbing you while you panic or clean yourself up. On some Asian beaches, fake drownings are used. You plunge in to help and your belongings are rifled.
It's best to be aware of what's going on around you in any public place and to be a little suspicious of strangers who appear to be trying to single you out. If you are the victim of a minor assault, suspect that it's the prelude to a robbery attempt and if you feel safe enough, try and get in a position where you can look after your belongings. Unfortunately you may need to refuse the help of concerned onlookers; it's common to have an accomplice pose as a concerned onlooker.
Sexually attactive people are a fine distraction, and conspicuously available ones even more so. However, sampling the local streetwalkers puts you at risk of crime. Prostitutes can be used as bait for a variety of scams:
Even if you do not allow them to lead you anywhere, streetwalkers can be dangerous. A person who brings one to his hotel is quite likely to miss his watch or wallet in the morning.
If you are willing to take the health and legal risks of hiring a prostitute, go to a "massage shop", "sauna" or whatever the local euphemism is. These establishments are significantly safer than the street workers.