Delia Sherman is the author of fantasy and YA novels such as The Magic Mirror of the Mermaid Queen and The Freedom Maze.
Talking through whatever I'm stuck on with a writing buddy. Frequently, just describing the problem I'm having with plot, character motivation, whatever, will start to give me ideas. And if it doesn't, the questions/comments/suggestions the writing buddy gives me will almost always shake something loose. You just have to remember that other people's advice and alternate ideas are best viewed as prompts, not directives.
Imagining (or talking through or writing) a difficult scene from the point of view of another character. It's particularly useful to run scenes from the antagonist's POV. Keeps you from writing simple Evil Nasties.
Well, I was a beginning writer rather than a young one (when I was young, I was told to write about my real life, which kept me from writing fiction for many years). I guess the best advice I got was from my writing partner Greer Gilman, who encouraged me to push past my first ideas (which tended towards the conventional and expected) to what I really wanted to say.