If you want to be a successful songwriter, you need two elements. Well, everyone's got an opinion or a point of view. And care to hear it over and over an infinite number of times. You figure out that second element by listening to the greats. A lot of people do it by shear numbers, figuring if they write dozens or hundreds of songs, they'll eventually have a hit.
Some people discipline themselves to write nine to five every day.
Persistence is certainly part of it. I've got songs sitting on the shelf that I think are better than the ones people have recorded. I hope to find a home for them someday. As a songwriter, you hope that you'll eventually have an audience for your song. That way, maybe you cut to the chase quicker.