There are books with label contacts, manager contacts, songwriter contacts, etc. It also says if they accept unsolicited or solicited material, or both. The best way is to get at somebody's manager or label, obviously. Call 'em up and ask how you would go about doing such and such. If they only accept solicited material, make sure you call them and tell 'em who you are, what you're sending, and when you're sending it. You'll get a name of the person you're supposed to address it to.