I often find that I can get into the flow of my writing better when I play music that fits the mood of the scene I'm working on. If my characters are angry or something sinister is going on I will listen to music that invokes those types of feelings. If I'm writing something more subtle I may listen to Enya, The Cranberries, or some of my Celtic music collection. Sometimes the randomness of internet radio can help as well. I have found Pandora to be a great free way to discover new artists based on my musical preferences. You can create separate "stations" and through an intuitive like or dislike system you can refine the music that Pandora will look for as it searches the database for like sounding songs and artists.
Sometimes I just play music on the surround sound speakers hooked up to my computer but other times I need to drown out my thoughts, so out of respect for those around me I put on headphones and turn the volume up.
This often helps me focus on my writing and gets me in the zone. I know I shouldn't be thinking about anything else so I just let the music carry those random thoughts away and focus on the subject at hand. I imagine I'm standing alongside my characters, I try to feel what they are feeling, see what they're seeing.
Aside from Pandora, I find it also helps to have a good music collection of your own. Sometimes I have specific songs that I want to hear. For those times I find it much more convenient to have a playlist ready with my favorite songs.
How about you? What are some things that you enjoy doing to help you get in the zone?