Thai people know that foreigners do not understand Thai culture but it doesn't hurt to try because some traditions when broken are offensive.
Temples and their objects are sacred. Often you will read that you should take off your shoes when entering a temple but you should also take off your sunglasses and hats. And ladies, if you want to show some real propriety, wear something white. Most importantly women must not touch a monk, do not offer your hand, or touch his robes. If you wish to give the monk something you must first hand it to a man who can then hand it to the monk or place it on the floor in front of the monk. Taking photos is allowed but not of anyone not fully clothed and this includes the grounds surrounding the temple. Always ask if it is ok to take photos - Khor thailuup khrap or ka. You should also wear suitable attire, shorts, vests, and mini-skirts are ok but no bikini tops etc. Monks, Phra, are educated and respectable men, they like to talk to tourists so if one beckons to you, approach the last two paces on your knees. When you stop, bow your head and make the sign of Wai high up with your thumbs on your nose. This isn't crucial for non-Buddhists but it shows respect. If there is a money pot nearby put 50 or a 100Baht in it. You must not climb on any object and if visiting ruins you should ask if you need to remove your shoes or if it is ok to climb steps. Statues on these sites are also sacred and you must not climb on them. If this seems a lot to remember then please do not visit any temples during the morning.
Really importantly - If you buy any Buddhas to take home make sure they are not damaged. It is a criminal offence to smuggle broken Buddhas.
Couples (and that includes short time girlfriends) holding hands in public is ok but no more than that. Kissing, cuddling, walking down the street with arms round each other is taboo. Whilst putting your hand on a girls bum is deemed rude it would be considered amusing compared to touching someone on the top of the head so try not to be overfamiliar with Thais in public.
Thailand does not have nudist or topless beaches and it is not acceptable to go to the shops in a bikini. Be warned, if you overstep the mark you will be arrested and whilst Thais are considerate to tourists the Police are not.
Whilst men will see women use the cubicles in their toilets it is not alright for men to use the ladies. Strangely it is ok for men to urinate in public provided they show a little decorum but it is not ok for women.
Treat women with respect and that includes bargirls, Phuh Ying Baah, hold the door open and give up your seat on a songthaew. Women are the master of their household, they are not, as some tourists seem to think, second class citizens.
Do not lose your temper, ever, it will achieve precisely nothing. Most of all, be polite and show your gratitude.
Thais do not drive like lunatics, they drive differently to us so make sure you understand the rules.
Thailand has a zero tolerance to drug abuse.
Break the law and you will see the inside of Bangkwang.
Do not think you can con' the Police with false insurance claims, they will catch you out and lock you up.
Do not put your wallet in your back pocket or backpack or leave bags unattended.
Do not join in the Muay Thai or dance shows.
When you 'Check bin', Geb Tung, check their figures - this is polite practice.
Always check your change. Always give a gratuity.
Thailand is a safe country but check with your embassy about border disputes etc. before backpacking.
Girls - Do not go to a private party with Thai men.
Men - Do not pick a fight with Thai men.
Thai women ARE NOT prostitutes and you can tell the difference with ladyboys despite your mates claims.
Bar girls are to be found in the bars. Proposition girls on the streets at your peril.
If you pick up a freelancer on the beach you will get a) Robbed while asleep. b) Mugged by her boyfriends mates. c) Herpes.
Keep your valuables with you or locked in a safe.