I write a lot of notes, and they're not very structured or organized. So basically, it's hell stumbling upon something I've written a while back and trying to figure out just what the hell it's supposed to mean.
One of the best habits I've picked up in the last few years is to always write a date in the upper right corner. It doesn't matter if I'm taking notes during a meeting, jotting down some thoughts on an upcoming lecture or scribbling a quick todo or memo on a post-it. Everything gets a date. (Okay, I'm a bit sloppy on the post-its, but I'm getting there.)
In many cases, I also write just what the heck it's about, like a one- or two-word title. For example, in the upper right corner of the current page of my notebook, it clearly says:
Obviously, it's notes taken today regarding my thesis. That way, even though they're a bit cryptic, I'll be able to put the notes themselves into context and understand what they mean, even years from now. To some extent, of course, and provided that I understand them now (hmm), but a lot better than without date and title.
So, in short: always put a date and a title on everything.
Simple, but powerful.