Many Thais pronounce every s as a th (as in thin) and cannot pronounce an /s/.

thin and sin, sick and thick, sing and thing are pronounced the same.

I don't know whether this is a regional thing.

They seem to have no problem saying "thaw theua" instead of "saw seua".

But it becomes a problem when they want to learn English,  Spanish or any other language where a /th/ is not an acceptable substitute for an /s/

 

Today I tried to teach a Thai friend an /s/, with little success. 

 

Any experiences from the English teachers?

This may help.

There are a couple of old pronunciation books names 'ship or sheep'.

They have written text and accompanying CD's.

They also have diagrams that show the face or mouth and how the sounds are produced. :)