I'm in advertising, and we have a phrase that became painfully relevant to me after I attended my first writer's group meeting: "If it's on the rail, it's for sale."
In my day job, that means if you take a TV storyboard/print ad layout/logo, etc. into the conference room, put it up on the "rail" (a small shelf running the entire length of the wall), it's fair game to be praised, derided, bought or trashed. Every once in awhile, we'd throw in a decoy idea--one that was so laughably lame that the client couldn't help but see how superior the recommended idea was. Naturally, they'd buy the lame one, and when we protested, they said, "then why did you show it to us?"
Same principle here. I crammed my book with lots of hilarious backstory, pages of tangential anecdotes and entire scenes of throwaway comedy. My writer's group had the effrontery to point all this out to me. They'd seize on some inconsequential detail buried in the backstory and go on and on about how it didn't make sense, that the taillights on a 1966 Mustang pointed out, not in as I described, and so on. Meanwhile, I'd be thinking, "Why are you so fixated on THAT? That doesn't even matter to the story!"
But if I didn't want them to comment on it, why was it in the book? And if it didn't matter to the story, why was it in the book?
And that, my friends, is how I was able to take a 184K word novel down to 109K words. I'm not saying it wasn't painful. And I'm not saying it happened overnight.
But I am saying that until I did it, I didn't get an agent.