Instead of throwing out my notes, I tend to "refactor" them multiple times, condensing them down to the bare minimum. By the time I get them to that point, I no longer need them. I also tend to utilize multiple mediums: first note taking is writte; then I condense them to a digital format; I print that off and highlight, work in the margins, etc.;I will sometimes rewrite them a final time, depending on difficulty of the material.