Once more, some very good counsel for us ink-stained scribblers:
What Not to Write, Part 1

Over at Smart Bitches, Trashy Books there is an ongoing thread advising romance authors what not to write. Since urban fantasy can often have elements of romance in it, I took their advice about ‘werepanther spooge’ under advisement.
Writing Urban Fantasy
TV Tropes has some advice on writing urban fantasy.