David Foster Wallace on That/Which/Who/Whom

For writers, the abstract rule that that introduces restrictive elements and which introduces nonrestrictive elements is probably less helpful than the following simple test: If there needs to be a comma before the rel. pron.,  you need which; otherwise, you need that … Misusing that for who or whom, whether in writing or speech, functions as a kind of class-marker—it’s the grammatical equivalent of wearing NASCAR paraphernalia or liking pro wrestling.
- David Foster Wallace