If, say, a left of center magazine or some other Washington institution wanted to engage in a bit of class-based affirmative action, I have a fairly simple suggestion. Just make sure you reach out beyond elite schools. I've attended and taught at a variety of institutions, and some excellent students can be found most places. And while I don't know the hiring practices of random left of center magazines, or for Congressional staffs, or for the Washington Post, it wouldn't surprise me if first round resume weeding is frequently done based on the college the applicants attended.
I'll second that. Sure, the East Coast centrism of opinion magazines is easy to understand, since they're almost all based on the East Coast. But while I can't say for sure that things haven't changed recently, a few years ago I was noodling around on this subject and was astonished at the hegemony of the Ivy League in the mastheads of most progressive magazines. I expected it to be heavy, but my recollection is that my (admittedly unscientific) sample was something like three-quarters Ivy League. Considering the number of top notch universities elsewhere in the country, that's pretty hard to defend.
So yeah: recruit on the West Coast. Lots of smart liberals out here! And at public universities, which might produce a wider range of sensibilities. It's true that East Coast weather sucks and us Californians are more than a little crybabyish about snow and sleet and whatnot, but Ezra Klein managed to make the transition. I'll bet plenty of others can too.
UPDATE: But not just California! Recruit from all the other states too!