I am often guilty of saying "Pass the po-tay-toes" at dinner. And at Disneyworld, when we entered a ride we'd yell
"Into the miiiiiines!"
Other than that, I don't really quote any one line (from either the books or the movies) all that often -- it's usually just whatever comes immediately to mind.
EDIT: Wait, just thought of one. I sing
"Old Tom Bombadil is a merry fellow, etc., etc." simply because it irritates a friend of mine. She loves LotR but finds Mr. Bombadil very annoying.