It's no secret I'm a huge Harry Potter fan. Some people can read that series and come away thinking they're books for children.
And yes. Harry and his fellow students grow as the series grows. But even in those early books, there's a depth to the storytelling you might miss if you only read the series once. Before the release of each book, I'd reread the books that had been released. With the new book's revelations, I would find the way I looked at the earlier books had changed--and I'd have to go back and read them again in the new light.
I would love to be able to write with the kind of depth Jo Rowling did there--stories that appeal to children while providing adults with a rich adventure as well. And believe me there are millions of adult HP fans. On the Leaky Lounge, we had doctors, lawyers, geneticists, writers, professors, physicists, etc. arguing back and forth about whether or not Dumbledore was dead! The caliber of the debate was amazing.
Author Dan Wells touched on this last October when he was the guest speaker at the UVU Book Academy. He said he's frequently asked this question, and he said ideas are everywhere. Don't just look at a news report as it is but let your imagination play with it. Do a lot of "what ifs". He said he was watching a science show on PBS one day, and one little tangent triggered by the story led him to his new series.
Let your imagination out of the box. Pull unusual combinations together, and see if you can make them work.
Like in Star Trek, you can let your imagination "boldly go where no one has gone before" (even if that is a grammatical no no).
So where do you get your ideas from? Do you have a process that you use? What kinds of things trigger your ideas?