I've gotten more song ideas in the shower than any other place. I know it's a cliché...but for some reason it just happens, and it's not something I consciously think to do. I'll just be moving along, and then a melody pops into my head. I don't know if it's anything to do with the sound of the water which kinda masks all other outside noise, that makes it easier to hear a song in my head.
There were times when I walked out of the shower more than once with just the towel around me...gone into my studio and stood at the piano working out what I was hearing in my head...because if I didn't, by the time I would dry and dress...it would be gone, unless I kept singing it over and over in my head the whole time I was getting dressed.
Another oddity...I've heard songs while running the vacuum. It's like that drone from the vacuum drowns everything else out, and suddenly I hear a song in my head. It's the oddest thing...but it seems like the times when I am least trying to think of a song...that's when they come, and it's often when I am engaged in some semi-subconscious activity, like showers or vacuuming, or even just driving in the car (radio off), and that drone of the car and the traffic...it puts me more "in my head"...and that's when I get song ideas.
AFA storyboarding...that's common with film/video so that all the players/techs can get the general idea of the story line, which is much harder when you're just describing it with words. I don't know if it would be of much value of one person when writing a song. I think you will get more mileage by simply writing down some phrases or thoughts about anything...life, love, the world...just jot them down, single words or short phrases without pause to think about them.
Then afterwards, sit and ponder each one, and see if it triggers some further song ideas.
I usually think of a "song story" off of a single word or single phrase, and then use that to build on...rather than first laying out a complete story from start to finish, and then writing lyrics to it. Like for example - "She woke up to a dark morning"...it's just a random thought, it may not even end up as part of the song lyrics, but it can trigger the ideas for the story and the lyrics.
I find that often once you start, the lyrics lead you, and when you follow the rhyme and meter you set, that also pushes you along, or sometimes forces you to change your direction to make it all work...which brings me back to a point in another, recent thread...the more words you have at your command, the bigger your options for where you can take the lyrics. If you limit yourself, it may seem like it's easier, but you then have less creative options.