Do you ever make up new words or phrases in your writing?
Since my middle graade novel involves a made up place and an invented scenario, I'm also creative with the language my characters use in dialogue. So far, most of my invented words are for emphasis -- the equivalent of "Wow!", "Really?" or "No way!"
Another way I use invented words is for some character names. It's easy to do by taking an ordinary name and changing it a little until it sounds right. Using invented names is one signal to my readers that the world of the novel is not quite the same as the world we live in.
I find that one of the tricks to using invented words or expressions is to balance their use. If they are too infrequent, they won't seem like part of the everyday lives of the characters. If they are overused, then I think they'd be distracting for a reader. I also try to make their meaning clear, so a page of translations won't be required.
P.S. The League of Extraordinary Writers, a group of YA writers of science fiction and dystopian works, are having a cool contest with giveaways of their new books. You might want to check it out!