(A larger version can be seen here.)
I came across this interesting item at Wikipedia. It's a poster advocating how Thais should and should not have dressed during the conservative Phibulsongkhram era (1938-1945). It reads:
Thai Culture
When in communal or official places
When in public or on the street
(left side)
Do Not
Dressing in this manner is not part of Thai custom
Men:
Do not wear a sarong or expose the upper body
Do not shave your head or wear a Muslim-style hat
Women:
Do not expose the upper body or wear a shawl
Do not wear only underclothes or carry things on your head
(right side)
Do
Thai custom entails dressing this way
Men:
Do wear international-style clothes or Thai style short trousers
Do wear a proper button-down shirt
Women:
Do wear your hair long and a clean, proper outer shirt
Everybody must wear a proper skirt