(1) Don't write your autobiography. Nobody cares. It's better for your story to be the biography of someone the reader would like to be.
(2) Jump right in and start in the middle of the action. Don't "warm up" the reader with background information.
(3) Do not allow adult adult characters to take over the story - at most they should facilitate or, better yet, impede, a young main character.
(4) Avoid sentimentality - keep an edge in your work.
(5) Your character must grow / evolve between the first page and the last. They have to take some sort of independent step towards maturity.
(6) Read. Read. Read. Read. Then read some more.
(7) Research. Research. Research. Research. Then research some more.
(8) Once you have an audience of young readers, do NOT correspond with them via e-mail. E-mail says you don't care enough to send your best.
(9) Avoid adverbs.
(10) Improve your vocabulary.