All I can say is, "Wow."
A few weeks ago I pulled out my print-out of the very first book I ever wrote, Angel's Grace, and went through it with a red pen.
What was I thinking?! I slashed thought that thing like there was no tomorrow. I couldn't believe how many things needed to be fixed. I'd actually sent a query letter to agents for it?!?! It is no longer a surprise to me why I never had a single request for it. It was AWFUL!
Well, not awful per say. But it reeked of a seriously inexperience writer. I still love the story, the plot was great, and the characters were well thought out. It was the wording, sentence structure, descriptions, and the voice for each character needed more personality.
I wrote Angel's Grace over two years ago and since then I have learned so much about writing. Through reading blogs, reading what agents are looking for, twitter, reading more current Y.A. novels, and from simply writing and finding my own style, I can't believe how much I've improved as a writer.
I cannot tell you how important it is to take a step back from your book before you send it out to agents. Not necessarily two years like I did, but enough that when you read it it feels like you are reading another writer's work. It is hard to find your own mistakes when you have read something a million times. The lines simply start to blend together.
And don't even get me stared on my query letter. Towards the end of it I'd actually put, "This is my first YA novel." Bwahahahaha!!!!
In short: Write. Try different styles until you find your own. Read. Read blogs, read about writing, and read what you want to write.