I was thinking about Don's "No music with lyrics" rule. I find that music helps me a great deal. Generally I have just sort of a background stream of click-hop music playing. I've always been a fan of electronica and IDM in general - click hop sets a pace for me and is usually "background" enough that it doesn't take me out of the work.
This is what I'm talking about if you're unfamiliar with click hop. It sounds like tennis shoes in a dryer to some but I like it.
But sometimes you need something that sets the mood. To this end I highly recommend soundtrack albums - the more emotional the movie or similarly tonal the better. I really liked the "Me And You And Everyone We Know" soundtrack because it's the sound I like and it has emotionally charged songs on it.
Every now and then I'll break the no lyrics rule and put on a specific song because the words have relevant meaning or the mood is just right. Usually though I put that on and take it off as soon as it's over. It's too hard to write dialogue when someone's talking to you.