Bring an open mind and a sense of humour. Don't come with too many preconceived ideas about what Thailand is like, as media and friends’ experiences have a habit of distorting reality.
If you're sticking to major cities and tourist areas, don't worry too much about under-packing; you can get hold of any essentials you've forgotten. Essentials are a swimming costume, a day pack, an umbrella in rainy season and some warm clothes if travelling in Oct-Dec, as some areas get cool. Some sources say there is no point in bringing a raincoat during the warm rainy season because it is so hot and sticky your raincoat will be uncomfortable.
You will only need a couple of changes of clothes as you can get washing done anywhere cheaply. Sandals for when your hiking shoes are too hot can be bought cheaply in Thailand, although large sizes for women are harder to come by. If female and anything above a size 2 (US), size 6 (UK & IRL), size 36 (rest of EU), busty, or tall, it is often difficult to find clothes that will fit you in any of the Thai shops. If you are male and have a waist more than 38" you will have trouble finding pants. You will largely be limited to backpacker gear (the omnipresent fisherman pants and "Same Same" t-shirts) or Western imports in Bangkok malls, for the same prices as back home or more. While laundry is cheap, it is useful to bring a few changes of clothes, as you may sweat your way through several outfits a day in the Thai weather.
Take enough padlocks for every double zipper to stop wandering hands and lock up your belongings, even in your hotel room. Lock zippers through the lower holes, not the upper ones on the pull tabs — although even this precaution won't help much if you encounter a razor-blade artist.
Take snorkeling gear or buy it on arrival if you plan to spend a lot of your time in the water. Alternatively put up a notice looking for gear from someone who is leaving. A tent for camping if you are a national park buff is a good idea, as is a compass. You might like to bring compact binoculars too if wildlife is your thing. A good map of Thailand is also handy.
Take earplugs for when you're stuck in a noisy room or want to sleep on the bus. Take a mirror for shaving, as often budget places won’t have any. String is very handy for hanging up washing. Cigarette papers can be difficult to find, except in tourist centres. Climbing shoes for rock climbing are useful as Thailand has some of the best cliffs in Southeast Asia.
If you have prescription glasses, it is a good idea to bring a spare pair of glasses or contact lenses plus a copy of your prescription. Bring a book you're prepared to swap. A personal music player is great as a huge range of cheap music is available everywhere.
Into the toiletries bag throw sun screen and insect repellent. Mosquito coils are also a good idea. A small pocket size torch or flashlight will come in handy when the electricity goes out or for investigating caves. Passport photos come in handy for visas.
If you plan to travel long distances by motorbike, purchase a good quality helmet, which you can do in Thailand. Last but not least, pack your stuff in plastic bags to stop them from getting wet, especially when travelling in the rainy season or on boats.
Aside from the above, the following are recommended: