My story is far too long. What do I do?

A writer recently howled on a forum that his novel was far too long: it was 180,000 words when people were saying that no agent will look at a book over 90,000. He did sense that there were things to cut, but didn't know where to begin. And how on earth was he to get it down to half the length - lose every other word, effectively - and still have a novel, not a blood-sodden mess? There are an awful lot of writers who have faced up to this problem, but actually it's two relatively separate problems: what length...