A few things (serious and silly) that I picked up from YallFest Panels:
1. Root imaginary worlds in reality. Give your readers some signposts - it helps them connect to it.
2. Setting and landscape are super important, but don't forget about the time frame in which your story takes place. Time of day, time of year, decade, century is all important to keep in mind (even if it doesn't make it into the pages of your book).
3. Really bad writing comes from trying to write yourself: When we try to write ourselves, we're really not. We lack the distance and the result is something too perfect, or too self-debasing.
4. The usefulness of the slush pile is dwindling. Fewer and fewer books are making it out without being shredded. The solution? An Agent. It's becoming increasingly important to find an agent.
5. Finding an agent is like dating - find one who likes you and believes in your work. You don't want to go into business with someone who doesn't 'get' your work.
6. Immature poets imitate. Mature poets steal.
7. Carrie Ryan: The best weapon you can have in a zombie apocalypse is someone who runs slower than you.
8. If you're going back into history, take advantage of it. Go into the bigger cultural world.
9. Make sure your villain believes in what he/she is doing. Even the people you are vilifying need to make sense.
10. If you're stuck moving the plot forward, get out of your protagonist's head and enter your antagonist's.
11. Empathy is important: It's difficult to empathize with losing an entire nation, but very easy to feel for 2 or 3 people.
12. David Levithan: Writers don't peak, we just keep getting better with age (and experience).
13. Kami Garcia's fail-safe: Whenever the book is not working out you just need to add more making out.