Just ran across a nice summary of when to use which and when to use that … which I always find hard to explain. From the Dayton PRSA website:

The difference between “that” and “which” can be vague until you get the hang of it. “Which” sets off parenthetical information: stuff that can be lifted right out of the sentence without changing the sentence’s main point. “That” introduces information essential to the main point of the sentence. “The press conference, which will be held at 2 p.m., will include a question-and-answer session.” “A company that listens to its customers will succeed.”

Thinking of which clauses as parenthetical information — bracketed by commas, rather than parentheses — is a nice way of explaining this distinction. And we didn’t even have to use the words restrictive and nonrestrictive, which tend to make non-editors’ eyes glaze over …