Writing group today reminded me of a mistake I sometimes make - which is feeling like I must perfect a piece of writing before I show it to anyone. Now, I'm not saying you should send off an unpolished draft to an agent or editor, but sometimes another opinion, a fresh pair of eyes, can help to resolve a problem in half the time.
Another mistake middle-grade writers sometimes make is only getting feedback from other (adult) writers. Kids make wonderful advance readers and can sometimes put their finger on a problem that grown-ups miss.
Now I'm off to convince my nine year old and her friends to read another draft of my new book!