I'm not sure if this is true for anyone else, or if it's just my funky brain chemistry, but there are certain kinds of music which really help me focus on writing and get my creative juices flowing.
Apparently there is anecdotal evidence that classical music, aka harmonic music, helps the brain relax and therefore more easily absorb information, while dissonant music increases your brain's "jumpiness" and therefore makes you more inclined to energetic exertion. This partially explains why you don't hear hard rock in elevators and classical music in gymnasiums.
I have never, ever gotten classical music to work for me. Either it does nothing, or it relaxes me too much and puts me to sleep. Either way, it's not conducive to concentration.
What does seem to work for me is a kind of half-step between the two: techno or electronic music that features low, regular beats and a deep, rich, "dark" sound like you might hear from a choir of male voices. I don't know what it is, but this mixture seems to hit the resonant frequency of my brain's creative center, and it really helps me to tune out the world and concentrate on my writing. Plus, the more I listen to it the more I associate it with writing, so through Pavlovian conditioning my brain thinks "Okay, time to get serious and write" whenever I hear it.
Some samples of what really works for me:
Pretty much anything from the Tron: Legacy soundtrack, but especially "Recognizer".
Similarly, just about anything from E Nomine, but "Mitternacht" is my favorite.
And the first seven tracks (basically, DJ Miss Lisa's set) of Hot Import Nights.
Track 7: Hot N Tots (click to hear; no video exists)
So that's what works for me. Any other recommendations?