Poem clusters, song cycles, series, families of related poems: whatever you call them, they can be helpful in breaking up a longer work into palatable pieces. You can take one
idea and develop it by breaking it up into shorter, related poems or songs. Sometimes this works well if there are multiple points of view. You can write one poem from the point of view of one person or
character, while the next poem is from the point of view of the other. You can separate events that are separated by time or place. That way, you can keep the Classical Unities for each poem.
You can write a series of poems that are the continuing adventures or saga of a character or characters. Think of it as rhyming soap opera.
Another possibility is just to take a theme and write multiple poems on the theme. You can take a loose cluster of poems or songs about a single theme and tie them together with a bridging song or a song to
put them in context. Tom Russell used an excellent example of the bridging song in his The Man from God Knows Where album. He did this with two songs/themes. The first was the song, The Man from
God Knows Where. The second was a theme of The American Primitive Man.