I admit it. I'm just a bit of a whore for mood music when I'm writing. It puts me in the right frame of mind, immerses me in the vibe I'm trying to convey in my descriptions of the visual at hand.
For instance, I just finished writing a scene where three rough-and-tumble Angel foot soldiers are responding to Fallen mischief, engage in an urban gun battle and unfortunately fall victim to a trap.
So I use a playlist very strategically designed to keep me in a dystopian urban action mood.
Like Cold. And Linkin Park. And Killing Joke.
And for a healthy dose of apocalyptic imagery along with the dark urban alternative music, a little Gary Numan.