A song should have a beginning, a middle and an end. A hook is the main idea of a song, and usually, but not always, is the song's title. Remember that songs generally are either a dialogue between two people or a narrative simply told. After you've written your song, look at it all. If you wouldn't have said it naturally to a lover, friend or enemy the way it's written, then it probably should be rewritten.
Poems make very boring songs.
When you're stuck, try another angle. This tip works often enough to make it one of the most valuable tips ever given to me. Know when to quit. Finally, never overwrite. After you've told your story, hit your hook and get out.