I've got the beginning of my story and the end pretty much thought out, but it's hard to write the biggest part, the middle. Any advice on how to come up with material for the middle of the book?
That's a big question The middle of the story is everything that leads up to the end (I'm being Captain Obvious today). Whatever changes your character to make the choices he makes in the end need to be included. For example (to be cliche): you have a student with poor grades at the start of the book, but he's the best in his class by the end. What makes him change? A parent dying? A new relationship? It could be multiple things. Show how the character deals with that journey. Personally, I have parts which I know must happen, but I let the story flow freely to those points. It doesn't always work -- I might have to go back and recreate some events between things. But it's at least a better structure than having nothing planned for the middle.
I would suggest working backwards - you know how the story reaches its final climaxe so ask yourself what happened immediately before this? or what could have happened earlier for this bit to happen. Not great advice, but that's how I'm working my YA novel at the mo' Good luck!
One thing I learned was develope your characters... dont have them exactly the same at the end as they are at the beginning. Show a journey of some kind, whether it;s spiritual, emmotional or whatever.
This is the best advice I've ever heard for developing the middle of a book. If you're truly writing about your characters, they are going to show you where the middle of your book goes to end up at the ending. Other than that, I'd suggest loosely outlining your plot on paper - major events, minor events leading up to those events, etc. Then you know what needs to happen to get to those events.