Kudos to the posters for some excellent info. My experience is rather dated at this point, but for what it's worth:
You might have better luck, for Americans at least, at the consulate in Chiang Mai, being a little further from the party zone, though I suppose you'd have to justify being there somehow.
We had a joint bank account with a decent sum in it for a while, which seemed to make a difference.
My GF had traveled within Asia, and her "experienced" passport might have helped us as well.
Like the original poster, we also made a point of appearing comfortable as a couple. We made sure that the interviewer understood that I spoke Thai, as well as her speaking English.
She made it perfectly clear to the interviewer that she was a proud middle-class Thai, and had very little interest in the U.S. beyond tourism. I suppose they hear that all the time, but she was like "you've got to be kidding - why wouldn't I want to come home?".
A pair of glasses and the right clothes won't hurt.