Brainstorm words or images about your story or theme. In a notebook dedicated to your idea or story, write down every word, idea or song that springs to mind or sketch every image or stream of thought you can think of about your idea. Just let the ideas flow.
Don't stop to evaluate their worth. Keep writing down ideas. When you can't think of another thought, put it away awhile, but keep it handy. Often the most powerful or meaningful ideas will appear after you've put the idea away. You never know when that fickle Muse will show up . . .
Don't have a theme? Don't worry. They often appear when and where you least expect them!