That's a really good question, Deda. I'm guessing that people getting informed about the problem is probably a start. And it might help if people put pressure on the governments of the countries where the sex slaves end up, to pass better laws to fight it, and also to put more effort into enforcing laws.
But I think it's also useful for people to keep in mind that the problem of the sex slave trade is just one of the many horrors that women (and children) are subjected to all over the world, and for people to speak up against all forms of abuse of women and children, and also all forms of human rights abuse, wherever they are happening.
It seems to me that problems of this sort tend to get addressed much more quickly when a lot of people speak up about them. The recent developments with regard to the sexual abuse of children by priests being a good example of this.