For audio devices, you can squeeze just a little more time out of your charge by using in-ear headphones. I realize that sounds silly, but you get the same level of perceived "loudness" at a much lower volume.

Also, you can save some battery life by listening to smaller files. Memory read/write operations cost lots of energy - moreso than processor ticks, especially if your device reads from a hard drive. Consider OGG or VBR mp3 if your device supports them.