This happened to me just today...and thanks to this thread it saved me 7000 Baht on Rambutri Road Bangkok.
The guy approached me and told me he liked my vibe and I was a good natured person and that he was a Yogi from India was interested in helping me.
He told me things about me and about my nature that are not naturally very open to the public at first glance... and he told me he would be able to tell me more.
He led me down an alley and asked me where I was from and he pulled out a briefcase with pictures of gurus and yogis and showed me a picture which he claimed he was a disciple of one of these gurus..
Anyway... He used alot of tricks and was extremely mesmerising... To this point I still do not have any idea how he could have known the name of my mother, my girlfriend, how old I am and how many brothers and sisters I have. It is my guess he is using techniques such as what Darren Brown the famous "NLP" street hypnotist uses..
It happened pretty much exactly as Ian Wuk described and I was only lucky that I had lost my bank card and that I had to ask my girlfriend what she thought about me giving him 7000 Baht to finish his healing/fortune telling. These guys are very skilled fakir/conmen so watchout. Luckily I only lost 100baht. However the effects of his hypnotic tricks left me quite rattled afterwards and I still have no idea how he did it.
Thanks to all who have offered advice here... my Girlfriend immediately went onto the internet and found this thread and warned me strongly against dealing with this man.
Good luck.