See, I am a packrat. I don't like throwing things out or getting rid of things, because even if I haven't used them in years, I am afraid I will want them later. (Which does on occasion happen. E.g., I held on to my high school notes for a few years after I graduated. Finally threw them out. A few months later I came up with a story idea that related to something we'd learned and I thought, oh, I could look up my notes on that... Except they were gone.)
I am also a sort of packrat with my stories. My ideas spin off all sorts of interesting sub-ideas about subplots and back story and world building and so on, and I don't want to throw any of it out, so I squeeze as much as possible into the book. (This, I suspect, is part of the reason I ended up giving up short stories. There was just never enough room!)
So when I'm moving, and when I'm revising, I have to decide what's really necessary. I don't want clutter in my new home, or in my book. With the moving, it's relatively easy--have I used it recently? am I likely to again? do I absolutely love it? can it easily be gotten again if I realize I need it after all?
With the book, it's a matter of getting down to the kernel of the idea. What was it that excited me about this story in the first place? What pieces are necessary for that basic story, for those themes? What pieces help support them? What pieces go rambling off in other directions that, as much as I might like them, just distract from the heart of things?
It can be hard. Sometimes I get to thinking that a story has to have a certain element, just because it was always there--when actually taking it out would hardly change anything, or it could be easily replaced by something more suitable. But taking it down to the essentials, saying "what is this story I want to tell, really?" and imagining I can use only that, often makes it completely clear what I don't really need at all.