Stickies
I go through packs of sticky notes, and highlighters, like nobody's business. When I read through my books for class, along with highlighting text on the page itself, I make small notes and lists on a sticky note and affix it to the appropriate page. It makes it much easier to find things later on, since all I have to do is flip through the stickies to find the appropriate subject matter. It's one of the best, and most useful, ideas I've ever had. I wonder how many trees had to snuff it in order to simplify my academic life?