There's this thing you do after you write your book. It's called editing.
Then there's also revision, improvement, and death-by-red-ink, but I suppose that's a bit redundant.
I'm not going to lie: Editing is one of the worst things ever. Because of something called ego, it might also be painful. But you've got to just push your sense of pride away and accept constructive criticism, because you're going to need a fresh set (or two) of eyes to help you proofread.
If you're anything like me, you'll need to go through a lot of times and polish. See, even when I'd gotten all the technical and wording things to the point where I was fine with everything there, I learned something pretty unpleasant:
They make guidelines for MG and YA books. My manuscript was 7,000 - 27,000 words too long.
Even though this isn't exactly the greatest news to get, I figure it is a good thing for my book. Now that I'm pouring over it with the mission to delete anything I can (I mean, 27k is a lot of words), I'm finding all sorts of things that don't move my story forward -- so they kick the bucket. Even the little things like thats and theres make a big difference, just because they add up so quickly.
Here's your homework for the night: a few really helpful references from the Literary Rambles blog, hosted by Casey McCormick.
About tightening your manuscript --
Part 1: http://caseylmccormick.blogspot.com/2010/02/omit-needless-words-tighten-your.html
Part 2: http://caseylmccormick.blogspot.com/2010/02/omit-needless-words-tighten-your_20.html
Part 3: http://caseylmccormick.blogspot.com/2010/02/omit-needless-words-tighten-your_28.html
And now, the word count guidelines:
Okay. This is the part where you get back to your editing. Good luck.
You just might need it.