In my first book one of the short stories had a poem in it. It wasn't just thrown in for the sake of putting a poem in. The poem ended up being an important mechanism for one of the characters to deal with a nasty situation. Every time she encountered a scenario that ended up being unnerving she recited parts of the poem. I carried this through the entire short story up until the end. I found it an effective way of showing she was afraid of something. The poem calmed her down.
Sometimes I try to have my regular sentences come across as poetic in nature. Poetic prose if you will. That doesn't always pan out but eh, it sounds good.