I am not from Mudd, but I went to a talk by Maria Klawe on this very topic. I feel suddenly potentially useful. Warning: this is all only from memory. I thought I had the slides somewhere, but cannot find them. I'll email Maria, and if I hear back from her, I'll pass it on.

First off, here's the abstract for her talk:

Begin abstract

In 2006, much like at many other institutions, about 10% of HMC’s CS majors were female. At that time only a third of HMC’s students were female, but CS was an aberration. About 20% of the Physics majors and close to 30% of the engineering majors were female. Four years later 42% of HMC’s CS majors were female, exactly the same percentage as the whole HMC student body. This talk describes how the CS department accomplished this change.

End abstract

She emphasised that they tried to change how their first year program was run, with more cooperative group work. She was big on how they did a lot of work to try and get rid of the "macho" attitude among their undergrads in early courses - by "macho" she seemed to mean some sort of arrogant hackerish programmer attitude. She mentioned a bunch of mentorship programs for female undergrads, and programs to help undergrads get to conferences like Grace Hopper.

But! A couple friends and I were bothered by something else she said they did: they changed their undergrad admissions so as to admit more women to computer science in first year. Because they are a small, elite college they were able to do this without affecting the quality of their students, she felt. I thought that probably this was what made most of the difference, but that's only my opinion.