Gotta Catch 'em All
For this theory to function, you need to actively collect ideas. Get yourself a notebook, app, or voice recorder, and carry it with you wherever you go. Take note of anything you find interesting. It doesn't need to relate directly to your project or even to music, assume that most of what you collect will never be used 'as is'.
Of course, you'll find plenty of ideas by listening to and reading about recorded music. Note down things that you enjoy, keep up to date with your favourite radio or DJ playlists, and read music press, blogs, and websites to discover what other people think of the writing, instrumentation, and production of these songs.