Some Advice for White People in Fandom--a piece about how to write fanfic about nonwhite characters, if you're white, and avoid doing something stupid, offensive, racist, and bad-writery. Probably valuable even if you're not a fan, a writer, or white.
If you screw up anyway, here's
How To Apologize (in general, not just for saying something racist).
Thanks to
wordweaverlynn for pointing me toward these posts, via her link to
Dog Whistles and Insults (on understanding and acknowledging that you've just said something offensive, even if you didn't mean to).