Rethink: The Surprising History of New Ideas
1. Know at least a little bit, about a lot of stuff.
2. Know a lot about at least a few things.
3. Constantly trawl for new "stuff" (eg, read books, magazines, the web, watch documentaries, etc.)
4. Think about how to combine different bits of the "stuff" you have in your head. Take notes, write down lots of ideas, even if they're dumb at a first glance. Keep chewing on things, and eventually the penny drops and you see some connection between things that makes sense.
5. Related to that, and I don't do this well enough myself, is "spend a lot of time thinking about the kinds of problems people have".
I've read a few books on creativity / innovation / etc., but I'm not sure I can say any of them specifically helped, although a few of the ideas may have been useful without my being consciously aware of it. I read some of the Lateral Thinking book and I vaguely recall thinking it seemed somewhat useful. YMMV. https://www.amazon.com/Rethink-Surprising-History-New-Ideas/...
Anyway, I think there's some value in reviewing "out of date" literature on various topics, and look for things that seemingly died, that may be ready for you to pick up and run with.
Finally, there's this book: https://www.amazon.com/Intuition-Pumps-Other-Tools-Thinking/...
I bought it a while back, but honestly haven't had time to get to it yet. But if seems like something that you might find interesting.