Bangkok is best in the so-called cool season (expect a mere 30 degrees), from November to February. It starts to get hotter through February, typically reaching a 40 degree climax in April and May, before the rains arrive. It’s still warm, though wetter, from June to August. At the end of the rainy season (September - October), whole districts of the city can flood, and there is frequent disruption.
The eastern islands (Gulf of Thailand) are affected by the northeast monsoon from October to December, with the worst of the rains coming in November. Travel to/from the island can be difficult during these months. January to April is high season, with hot and humid days; May through September are cooler, with pleasant breezes spilling over from the southwest monsoon during July - September.
The east coast of peninsular Thailand follows the same pattern, but the October - December monsoon is much milder; and there is also a little rain and cloud spilling over from the western monsoon (July - September). But it is rarely very wet at any time of year.
The west coast and islands (Andaman sea) have peak season in November to February, when the weather is relatively cool (30 degrees) and there is little rain. From March it warms up, and then comes the monsoon season (May - October). The seas can get very rough and you can expect some strong but short downpours (especially in May, June and September).
The north mainland (e.g. Chiang Mai) is best from November to January, when it's quite cool (high 20 degrees) and clear. But it is very busy. In February and March, swidden (brush) fires make it hazy in the hills, and low waters rule out rafting, but it's still fine for city sight-seeing. March to May is hot and humid, with rains typically coming any time after the Songkran festival (mid April). June to October is consistently rainy, making trekking muddy work, but ideal for rafting.
Festivals and events in Thailand are particularly colourful and vibrant, and it can be worth planning your trip to coincide with one. The 2 biggest - book early, or avoid them altogether! - are Songkran (New Year) in mid-April, which is basically an excuse to throw water at complete strangers; and Loy Krathong (Festival of Lights) in November, when Thais send floats downriver with a coin, candle and incense to take bad luck away. You'll also need to book hotels in advance if you plan to be in Bangkok around Chinese New Year (which falls on the first day of the Chinese Lunar Calendar every year) or Losar (Tibetan New Year). During the rest of the year, handicraft shows, beauty pageants and boat races are also popular. Many are determined by the lunar calendar, so the dates change from year to year.
NB, please do not rely solely on this information for your travel planning.
From the UK: carriers include Thai Airways, British Airways, Qantas or via the Middle East with Emirates, Etihad Airways.
From Europe: try KLM, Lufthansa,Alitalia, Air France, Finnair and Swiss.
From the USA: with Thai Airways.
INTERNAL FLIGHTS: Try Bangkok Airways, Thai Airways and Orient Thai.
BY TRAIN: The most popular route is the overnight sleeper train from Bangkok to Chiang Mai, which is an atmospheric way to arrive in the north. Other useful routes include the line south from Bangkok past Hua Hin and Pranburi to Chumphon (for Koh Tao); and indeed on to Surat Thani (for Koh Samui) and Nakhon Si Thammarat, if you don't mind spending a full day on board.
BY BUS: There's a good network of inter-city bus routes, but they are rather slow and can be uncomfortable for long journeys.
WITHIN TOWNS AND CITIES: You'll see songthaews - pick-up vans with 2 benches in the back. Standard taxis are also available in Bangkok and the larger cities. Another option is the three-wheeled tuk-tuk. Finally, if you're desperate, you can hop onto a passing motorbike-taxi, whose drivers wear brightly-coloured and numbered bib-jackets.
BY BOAT: Among the islands and the coastal areas such as Krabi, the easiest way to get about is by boat, either by (shared or private) 'longtail boat', or on a larger ferry-type service.Ferries ply between the major islands and ports, including Phuket - Koh Phi Phi - Ban Laem Kruat (for Krabi) - Koh Lanta on the west (Andaman) coast; and Koh Samui (Nathon or Big Buddha) - Koh Phangan (Thong Sala or Hat Rin) - Koh Tao - Chumphon (mainland) on the east (Gulf of Thailand) coast.
tourismthailand.co.uk - the Tourism Authority of Thailand in Britain