I respect Ms. Ockler's intentions but must respectfully disagree with her prescription for increasing diversity in YA literature.
I think that you don't see many white YA authors writing with very diverse casts of characters because it's pretty easy for people in this country to live lives that don't have very diverse casts. Despite the diversity of the country in which we live, I think many many people live in bubbles in which they don't really know anybody who isn't like them.
I think that it's going to be difficult for you to write fully-developed, three-dimensional characters from another background if you don't know any people like that in real life.
I'm not saying every author should move to the city or anything. There are parts of the country where it's pretty difficult to find diversity, and I don't think living there disqualifies you from writing fiction or means that you don't have important stories to tell.
But I think that writing a diverse cast of characters out of a sense of obligation is going to end badly. Look no further than Ender's Game, where Ender's friendship with a black character is handled so clumsily that it's really painful to read. (Basically Card, who is a hooting loon of an anti-gay bigot who has issued veiled threats of anti-government violence and therefore should not be getting any more of your money anyway, imagines that the way white people can show they're not racist is by throwing the n-word around and making slavery jokes with their black friends. I am not making this up.)
I actually think it's worse to write stereotypical, inauthentic characters than it is to have a cast of characters that's not diverse.
We write to reflect the world we live in and to imagine the world we'd like to see (as I wrote so eloquently here). I don't know how anybody else's creative process works, but when I've written casts of characters who aren't entirely white, it's because it fits the story and the setting, but also just because in some indefinable way, it feels right. I don't mean right as in "the right thing to do," I mean because, at the risk of sounding precious, the characters tell me that they're not white. Or middle class. Or heterosexual. Or whatever. I don't know this for a fact, but I strongly suspect that this happens more often when the people in my life aren't all exactly the same.
So I guess what I'm saying is this: I'd recommend against writing a diverse cast if you are only doing it because you feel like you should and you're fundamentally uncomfortable with it. And I suspect that the more different kinds of people you get to know, the more different kinds of people you'll be able to write.