To echo a few other comments here, I think one big rule is you have to listen to a lot of different music. Love it or hate it, I check in with Spotify once in a while and browse stuff in the various categories across all genres to see what is hot. Sometimes I get profoundly depressed and other times I get inspired, and a melodic idea or vibe will pop into my head.
One thing I have noticed about music as opposed to other forms of writing is that you can't (or at least I can't) choose to sit down and write a good song. (A bad song maybe, but not a good one.) I can sit down and force myself to write a book (and it is about as fun as driving a nail in your head) but it can be done.
Songs don't work that way for me. I can't try and write them. They just have to come out of the thin blue sky. It is like you are at the mercy of the gods or something. But, listening to other stuff helps, and I also spend a lot of time auditioning BIAB demos because they teach you A LOT about chord progressions and styles that work. That's golden.
Finally, there is the act of observing and taking notes.
Once upon a time, I went for a walk after a concert with one of the most famous singers and band members of the 80s and 90s. We ended up sitting on the hood of a burned out Pinto for 3 hours shooting the breeze, and while we did so a very intoxicated guy in a cowboy hat came up and started to share his life story. This famous singer pulled a cheap K-Mart notepad out of his pocket and a Bic pen and asked the guy to start over--and then wrote down every thing he said--and the talkative drunk guy was more than happy to oblige him. The beginning of this story was that God told him to stop listening to Elvis and to go Alaska and work on the pipeline and listen to the Grateful Dead. The end of the story was that God told him to hitch hike back home across the United States, read more of the Bible, and start listening to Elvis again. The stuff in between was more dramatic than the Grapes of Wrath. When the songwriter was taking notes, I have never seen anybody write faster in my life.
Later, when the band's next album came out, I heard a lot of stories and lines that sounded awfully familiar.
I learned so much from that I can't even begin to say, but I think it is all about careful observation of people, careful listening, and note taking.
Paul McCartney's Penny Lane is about a real barbershop. You can google the roundabout he is talking about and also see the barbershop building I believe. He just wrote down what he saw. It still give me goosebumps.
Dansk
Deutsch - Windows
Español
Français - Windows
Italiano - Windows
Nederlands
Polski
Português
Svenskt
русский
Japanese
Simplified Chinese
Traditional Chinese
Korean