I have software that does a nightly back up to my wife's computer across the room. I back up to DVD ~once a month, just taking that day, back as far as will fill up 1 disk. I try to do it often enough that I get overlapping back ups, so no 1 directory is only on 1 disk - protects against bad media. I make 2 copies of this, one stays at home in a folder (which I can easily grab if I need to run out quickly) and one goes to work with me and stays in my office there. About once a year I'll do a full back up to DVD.
I'm up to ~40GB now, growing very rapidly (about 30 of that is just in the last year), so I may need to get a Blu-ray writer soon