Men - a pair of lightweight long trousers.
Women - a lightweight long skirt or dress.
This will get you into temples and palaces without a fuss and stop you looking totally like a tourist! If you forget to bring the right clothing, though, don't worry - it's very cheap to buy here.
Light cotton blouses, shorts, comfortable shoes to walk around easily.
Don't forget to wear long pants and protect your shoulders when entering a temple!
Miscellaneous: If you happen to visit a school (especially in the northern part of Thailand) try to get in touch with a school teacher. They always need stylos and paper for the children!
Luggage and bags: If u forget your t-shirt don't worry Bangkok have a lot of markets that you can buy one in the cheapest price in the world.