If you must be told...

From The Morning News' iPod Etiquette:

#6 Keep the volume moderate.
No one else should be able to hear your music. That constant buzz emanating from your headphones is only slightly less irritating than your tendency to hum “Like a Virgin” whenever Madonna comes on. Speaking of which…

#7 Don’t get funky on us.
Yes, we know you like music. We can see that it moves you. This is because you’re always moving—bopping your head, dancing, drumming, even singing along. Please, stop it. Otherwise, we’re forced to feign interest in your childlike enthusiasm for a song we can’t even hear. It’s exhausting.