Okay, I just lost my whole post. :( Let's try again....
How do you end your book? You want it to be perfect, but your ending just isn't coming to you.
I'll start with what I do because I never start a novel knowing how it's going to end. (What's the fun in that?) But I usually figure out how it's going to end somewhere past the halfway point. When I want to figure out my ending, I gather information:
My characters' values and personalities
How my characters interact with each other
My characters' goals
Then I need to think about my story. I think about what has changed in the story. My characters have changed, their goals may have changed, the setting may have changed, and the rules of the world may have changed. (Like in Harry Potter and the Order of the Phoenix. Umbridge changed the rules at Hogwarts, making Dumbledore's Army a pretty scary thing to the people in charge.)
The story (the beginning and the middle) has to matter. So these changes--what happened earlier in your book--have to guide you to your ending. Otherwise, what's the point in telling the story? Without putting these changes into your ending, you might as well just give a list of bullet points, rather than bother telling a story.
Finally, taking all of this into consideration, I come up with every logical ending I can come up with, including:
Good guys win, hands down
Good guys win, but with great sacrifice (my personal favorite)
The good guys win, but not what they were hoping for
The bad guys win, but it's not as much of a victory as they'd hoped
The bad guys win, but suffer great losses
The bad guys win, hands down
**And I always like to throw in: "The planet blows up, killing everyone." It makes me feel better knowing that's an option, even though I haven't used it. Yet.
Then I choose my favorite ending. The one that makes me smile, that ties up all lose ends, that is the most satisfying. If you don't have that happy, satisfied feeling about any of your endings, then I'd suggest you go back and look at your manuscript. Try to find all those little details that might push the story in one direction or another. It might be something you never intended to include. (Holly Lisle calls this "leaving toys on the floor".)
I'm afraid that's the best advice I can offer from my experience, so I'll leave you with some links. Maybe someone else has a method that works better for you.
Story Logic: A basic how-to for story logic
Hitting the Writing Wall (and how to break through it)
The Rockpile Theory of Plotting (short, but interesting)