The theory was that human brains aren't that good at dealing with "I need to remember to do that later". He argued that if you ever had two things to do, one's brain would start to be unhappy because one of those tasks wasn't getting done (because you were working on the other). Since the second task wasn't recorded anywhere one's brain would feel the need to constantly keep remembering it, and then feel bad because it wasn't getting done.
I started off well by using a stack of 3x5 cards to try to organize things (aka hipster PDA). But of course that was too low tech, so I started using some text files, and then like all the other systems I've tried, at some point I stopped looking at the text files and went back to trying to remember everything.
One of the most critical steps of a "time management system" is to look at it. So I'm back to the 3x5 cards on the theory that physical objects function as their own reminder that you need to look at them.
But to make my life a little more pleasant with the 3x5 cards, I got a little plum colored 3x5 card carrier and some "durable index tabs".
(And yes I looked up the color name).