I'd like to add something to this. I've fucked quite a few Thai girls in Bangkok and most of those came from building a social circle. I'd get in touch with girls via couchsurf (you can use any social networking website really) and arrange a meet up for a drink or ask if they had plans to go out at the weekend with friends and tag along or even go to the weekly CS meets. The girl you meet might not neccessairly be good looking but she'll have friends. Go out with them, don't be a typical Western sleazy sex tourist, show that you're good fun and they'll think you're great. You've got a reasonable chance of taking one of them home that night if they are drunk, if not get the number and arrange a solo meet up a couple of days later. You should be able to take her home this night if not on the first as she's away from the pressure of her friends and you've already proved you're a worthwhile guy. I've now got a list of girls as long as my arm on Facebook that I can meet up next time I'm in Bangkok for parties, company, rides and sex. Some of them I hooked up with were realtively well off with nice houses, good cars, plenty of cash to throw about. I had fun getting driven around, taken for lunch, beers bought, getting taken to parties. From meeting the initial girl I met through CS in BKK I met and fucked 4 different girls in the end at different stages. They didn't all know each other directly but were friends of a friend of a friend type thing. You will do so much better in Thailand if you have other Thais as friends and it can even help for hooking uop in clubs, you get seen by yourself or with other farangs you're thought of as a sex tourist but being with other Thais is instant validation.
If that doesn't work for you try approaching shop girls or waitresses (the non pro variety). Go to the shopping centres as ymg said but talk to the girls working too, they give phone numbers out very easily and most will reply. Set up the date. The waitresses in the bars (be careful to avoid the bar girl places) don't earn very much and love the chance to go out for the night with a seemingly wealthy Westerner, chat with them whilst they work, make them laugh. As many of these girls are fairly poor they live in cramped apartments with lots of friends and no air con, they'll do anything to spend the night in your hotel or airconditioned apartment so make sure you've got somewhere to take them, if all else fails there is Love Motels.
If that doesn't work for you try approaching shop girls or waitresses (the non pro variety). Go to the shopping centres as ymg said but talk to the girls working too, they give phone numbers out very easily and most will reply. Set up the date. The waitresses in the bars (be careful to avoid the bar girl places) don't earn very much and love the chance to go out for the night with a seemingly wealthy Westerner, chat with them whilst they work, make them laugh. As many of these girls are fairly poor they live in cramped apartments with lots of friends and no air con, they'll do anything to spend the night in your hotel or airconditioned apartment so make sure you've got somewhere to take them, if all else fails there is Love Motels.