I was recently asked for more detail on a certain user flow, but since it was a secondary use case we hadn’t budgeted for detailed renderings. Good storyboards stand on their own and there just wasn’t time for that.
Instead, I combined Ryan Singer’s see/do technique with a little bit of Interactive Sketching Notation. What the user sees is shown or described on top, and what they do is on the bottom. Their actions obviously affect what happens next and where they navigate.
You can also use this with non-linear flows; I could have drawn out additional frames for package contents and terms and linked to them if they were important enough to see.