I'd say keep your lists on an electronic organizer, and when an item is done... don't check it off, delete it!

But I seem to have gone back to paper, or forgotten about what's even on my Palm Pilot's todo lists...

Uhm, sounds like you may be a packrat. I'd say you need to make a point of being willing to throw things out... that receipt you'll never need again, that grocery list that you just used to purchase groceries you are now putting away, etc.

It's hard, because us packrat's have a fear of throwing away something that "might be important later".

Anyway, I don't think you are keeping the old lists as a reminder of what you've completed... I think you are keeping them out of the habit of keeping things that "you might need later", because you haven't made the conscious effort to decide whether you will really need it later or not. I.E., force of habit.

Ok, I'm rambling. Anyway, good luck throwing away your irrelevant lists and keeping only the relevant ones. :)