Take a book that's not SF. Imagine it as SF. What would it be like?
One of the games I enjoy is to take a novel I admire or outright love that isn't science fiction or fantasy and try to imagine what an SF&F version of that story would be like. The easy way to demonstrate this game is by way of an extant example: If we turned The Count Of Monte Cristo into an SF story, what would we have? Easy: Alfred Bester's The Stars My Destination.
The game becomes harder, but also more rewarding, when you turn towards other books which SF&F readers (or even SF&F authors!) might not know as well.