Thailand has more than its fair share of scams, but most are easily avoided with a modicum of common sense.
More a nuisance than a danger, a common scam by touts, taxi drivers and tuk-tuk drivers in Thailand is to wait by important monuments and temples and waylay Western travellers, telling them that the site is closed for a "Buddhist holiday", "repairs" or a similar reason. The 'helpful' driver will then offer to take the traveller to another site, such as a market or store. Travellers who accept these offers will often end up at out-of-the-way markets with outrageous prices - and no way to get back to the center of town where they came from. Always check at the front gate of the site you're visiting to make sure it's really closed.
Avoid any tuk-tuks in Bangkok. Tuk-tuk drivers might demand much higher price than agreed, or they might take you to a sex show, pretending they didn't understand the address (they get commissions from sex shows). For the same reason, avoid drivers who propose their services without being asked, especially near major tourist attractions.
Don't buy any sightseeing tours at the airport. If you do, they will phone several times to your hotel in order to remind you about the tour. During the tour, you will be shortly taken to a small temple, without a guide, and then one shop after another (they get commissions). They might refuse to take you back home until you see all the shops. On your way back, they pressure you to buy more tours.
Easily identified with practice, it is not uncommon in tourist areas to be approached by a clean cut, well dressed man who often will be toting a cellphone. These scammers will start up polite conversation, showing interest in the unsuspecting tourist's background, family, or itinerary. Inevitably, the conversation will drift to the meat of the scam. This may be something as innocuous as over-priced tickets to a kantok meal and show, or as serious as a gambling scam or (particularly in Bangkok) the infamous gem scam. Once identified, the wary traveller should have no trouble picking out these scammers from a crowd. The tell-tale well pressed slacks and button down shirt, freshly cut hair of a conservative style, and late-model cellphone comprise their uniform. Milling around tourist areas without any clear purpose for doing so, the careful traveller should have no difficulty detecting and avoiding these scammers.
Many visitors will encounter young Thai ladies armed with a clipboard and a smile enquiring as to their nationality, often with an aside along the lines of "please help me to earn 30 baht". The suggestion is that the visitor completes a tourism questionnaire (which includes supplying their hotel name and room number) with the incentive that they just might win a prize - the reality is that everyone gets a call to say that they are a "winner", however the prize can only be collected by attending an arduous time-share presentation. Note that the lady with the clipboard doesn't get her 30 baht if you don't attend the presentation; also that only English-speaking nationalities are targeted.
Another recurrent scam involves foreigners - sometimes accompanied by small children - who claim to be on the last day of their vacation in Thailand, and having just packed all their belongings into one bag in preparation for their flight home, lost everything when that bag was stolen. Now cash is urgently needed in order to get to the airport in a hurry and arrange a replacement ticket for his/her return flight in a few hours time.
 Robbery on overnight buses
Thailand is quite safe for tourists. However, there have been some reports about people getting drugged and robbed while traveling on overnight buses. To avoid this, steer away from cheapish and non-government buses, make sure you have all your money stored safely in a money belt or another hard-to-reach place and always check your money balance before getting off. Warning your travel companions about this danger is also advised. In case this happens, firmly refuse to get off the bus, tell the rest of the people about the situation and immediately call the police. It may not be possible to stay on the bus, as your refusal may prompt the staff to unload your hold luggage onto the street and then continue to drive the bus without your luggage, forcing you to disembark or lose it.
Thailand's age of consent is 15 but a higher minimum age of 18 applies in the case of prostitutes. Thai penalties for sex with minors are harsh, and even if your partner is over the age of consent in Thailand, tourists who have sex with minors may be prosecuted by their home country. As far as ascertaining the age of your partner goes, all adult Thais must carry an identity card, which will state that they were born in 2533 or earlier if they were over the age of 18 on January 1st 2008 (in the Thai calendar, AD 2008 is the year 2552).
Some prostitutes are "freelancers", but most are employed by bars or similar businesses and if hiring a prostitute from a bar or similar business, you will have to pay a fee for the establishment called a "bar fine". This entitles you to take them out of their place of employment; it does not pay for any bedroom gymnastics.
Remember that bar girls, gogo girls and freelancers are all professionals, who are far more likely to be interested in money you can give them than in any continuing relationship for its own sake. Cases of visitors falling desperately in love and then being milked out of all they are worth abound. Thailand has a high rate of STD infection, including HIV/AIDS, both among the general population and among prostitutes. Condoms can be bought easily in Thailand in all convenience shops and pharmacies but may not be as safe as Western ones.
Technically, some aspects of prostitution in Thailand are illegal (e.g. soliciting, pimping), however enforcement is liberal and brothels are commonplace. It's not illegal to pay for sex or to pay a "bar fine".
Thailand has extremely strict drug laws and your foreign passport is not enough to get you out of legal hot water. Possession and trafficking offenses that would merit traffic-ticket misdemeanors in other countries can result in life imprisonment or even death in Thailand. Police frequently raid nightclubs, particularly in Bangkok, with urine tests and full body searches on all patrons. Ko Pha Ngan's notoriously drug-fueled Full Moon Parties also often draw police attention.
Possession of cannabis (กัญชา ganchaa), while illegal, is treated less harshly and, if busted, you may be able to pay an "on the spot fine" to get out, although even this can set you back tens of thousands of baht. It's highly unwise to rely on this.
 Civil conflict
In 2004, long-simmering resentment in the southern-most Muslim-majority provinces burst into violence in Narathiwat, Pattani and Yala provinces. All are off the beaten tourist trail, although the eastern rail line from Hat Yai to Sungai Kolok (gateway to Malaysia's east coast) passes through the area and has been disrupted several times by attacks.
Hat Yai (Thailand's largest city after Bangkok and its Nonthaburi suburbs) in Songkhla has also been hit by a series of related bombings, however the main cross-border rail line connecting Hat Yai and Butterworth (on the west coast) has not been affected, and none of the islands or the west coast beaches have been targeted.
In September 2006, three foreigners were killed in bombings in Hat Yai. Some rebel groups have threatened foreigners, but while targets have included hotels, karaoke lounges and shopping malls, westerners have not been singled out for attacks.
Make a photocopy of your passport and the page with your visa stamp. Always keep your passport or the photocopy with you (the law requires that you carry your actual passport at all times, however in practice a photocopy will usually suffice). Many night clubs insist on a passport (and ONLY a passport) as proof of age. It is not required that you leave your passport with a hotel when you check in.
Carrying your own padlock is a good idea, as budget rooms sometimes use them instead of (or as well as) normal door locks; carry a spare key someplace safe, like your money belt, otherwise considerable expense as well as inconvenience may result should you lose the original. Also consider some type of cable to lock your bag to something too big to fit through the door or window.
Watch out for stray dogs as some can be vicious.