Usually, when I devise a new riff, or progression, I try to envision/emote what the music brings to mind. Then, try to find a relevant topic to write about. My issue is that vocal melodies and vocal dynamics escape me most of the time. Then I am left with a song that musically and lyrically fit... in a logical and rythmic sense, but no idea of how to actually sing it.
I have had times, too, that I will look through old writings from years ago that just "fit" what I came up with musically two days ago. So, I would say not to "trap" one-self in a box by trying to stick to a set-in-stone format. The song will be what its meant to be.
