Miscellaneous hints

Travel light (1)
There's no need to take huge amounts of luggage. Hotels in Thailand will do your laundry cheaply, if you do run out you can buy underwear and socks cheaply at any market, and there are supermarkets and pharmacists to take care of most other necessities.
Travel light (2)
Once you're there, there's no need to carry a huge bag of "necessities" everywhere you go. Yes, it's important not to get dehydrated in the tropics, but that doesn't mean you need to lug litres of water everywhere you go. Anywhere there are tourists, there will be someone to sell them food and drink, and even in the most overpriced tourist traps, the convenience of not carrying all that weight far outweighs the cost. And do you really need to have a Lonely Planet guide in your hand every second of the day? Maps are light, guidebooks are heavy.
Plan your journeys, especially in Bangkok
Make the best use of non-road transport options: river and trains. Pick up taxis that are already facing the right way.
Taxi meters
(Not for the non-Thai speaker!) It's approximately 25 km from the airport to the junction of the expressway and Rama IV. If the taxi meter says something else, you're being cheated.
Jai yen!