Facetious title, I know, but really. If you want more women in STEM you need fewer women in fields that they go into because of their already existing interests.
I would start, in fact, with the most unbalanced fields, like Elementary Education, Nursing, Women's Studies, and Social Work. We need far fewer women in these fields, proportionally. After that, we can talk about getting fewer female students to study social sciences, foreign languages, and English.
Since more women are getting college degrees, we need to re-distribute the majors, which means, essentially, taking them out of certain female-dominant fields (English, Biology), and re-orienting them towards the physical sciences, math, and engineering.
We get a kind of skewered vision in the Humanities, because feminist literary criticism is based on the idea that women's writing has not been valued. This is true, and the battles of the 70s were worth fighting. My undergraduate courses are 80% female, though, so I'm not that worried that not enough women are studying my field.
You cannot really suppress male interest in engineering and computer science, because those are the hard-core quantitative / technological nerds who are going to do those fields no matter what.
Non-facetiously, if the majority of women STEM majors are in biology, I have no problem with that. Scientifically-oriented women simply like biology better than they like other sciences. You can argue that they are socially conditioned to like this, but so what? I realized yesterday that physicians aren't considered STEM workers. So in a way the whole category is not defined very coherently in the first place.