Remember not to point your feet towards anyone when you're sitting (and definitely not when you go to see one of the wonderful statues of the Buddha that are in Bangkok). Bargain for everything, the Thais expect you to and they like it when you do but be fair. If you haven't tried it already, Singha beer is great with the food. And if you hire a car remember that you are driving in a country where people believe in reincarnation
Oh and accept that Thais don't appreciate the exuberance that us Westerners can exhibit (although that doesn't apply in bars or at the Muay Thai - and that's worth a visit too).
You'll love it though, the Thai people are very friendly and welcoming but be wary of those smooth types that want to be your friend (you'll find them outside your hotel) and show you around.