As a happy iTunes customer, I make a very simple backup of my tunes: I burn them to Audio-CD, print a booklet and put the CD on the shelf in my living room.
You can, of course, copy the iTunes library (~/Music/iTunes) to any backup device of your choice. If you change your computer, don't forget to unregister the old one with iTunes before you take it out of service. If, after a crash, that's impossible, you should be able to sort it out with Apple support.