Learn some elementary Thai before you go. There are several courses on CD available. Many people there speak and understand English, but a few words in Thai (even with rather mediocre pronunciation) will be useful at the markets, and you will be praised as the greatest linguist of all times ...
It's fun, too. I learned a few rudimentary phrases from a Lonely Planet guide before going to Phuket and it's such a wonderful way to engage with the locals.
Phom chawp Thailand!
"Do not force others, including children, by any means whatsoever, to adopt your views, whether by authority, threat, money, propaganda, or even education." - Ven. Thich Nhat Hanh
"Whether I were to preach in brief, Sāriputta, or whether I were to preach in detail, Sāriputta, or whether I were to preach both in brief or in detail, Sāriputta, rare are those who understand." (A I 333, Sāriputtasutta)