Whenever I talk to kids about writing and tell them it's the rewriting I enjoy most, they groan. I guess if you're in school, rewriting means copying your papers over. But to me, rewriting is the most exciting part of the process. When I'm rewriting, I feel most creative. I've got all the pieces to the puzzle and now I get to put them together. I go through four or five drafts of each book. (When I was writing Summer Sisters I went through twenty drafts!)
Read your work aloud! This is the best advice I can give. When you read aloud you find out how much can be cut, how much is unnecessary. You hear how the story flows. And nothing teaches you as much about writing dialogue as listening to it.