It's almost as though you need to get ugly women - if the women are too attractive, then they are written off as "booth babes". Which is kind of sexist in its own way.
It's one of the hidden sides of sexism in our society that annoy me. I try to catch myself whenever I find myself doing that, and have been pleasantly surprised.
A side effect of this is that those women that are attractive and intelligent tend to begin to hide it. So you get self-reinforcing behavior from society. Things like guys belittling smart women etc... The whole thing is annoying from the perspective of a guy that loves talking to non-white guys about programming or almost anything interesting. /squirrel banter mode off.
Case in point: Hedy Lamarr, who is probably more well known for her beauty than for her contributions to science (http://www.google.com/patents?vid=USPAT2292387)
Make them large and noticeable, on a lanyard probably.
The problem the OP faced was that people at the conference were generally sorting people into the general categories of "technical and worth talking to" or "just trying to sell me something" and would make the Bayesian inference that a random woman they came across was in the latter category.
I can see two solutions here, neither of which is very good:
1) In order to promote gender equality, everyone at a conference should avoid judging people based on their gender. This would be ideal, but it's hard to fight casual statistical inference.
2) The conference could make sure that the people selling stuff at the booths and the people attending the conference have similar demographic profiles, so a random woman you encounter at the conference is as likely to be technical as a random man. In practice, though, this would mean preferential hiring of men for the promotional jobs.
Really, we need an option (3) of "get more women to be developers" but that's hardly news, and not something that can be solved in short order.