The design sojourn blog makes the following suggestion to designers (one of ten tips for guaranteed concept development success) and I think it applies equally to writers:
"1) Never Throw Away Your Work: Keep everything, your thumbnails, sketches, and even sketch models. The real reason is that design is essentially intangible, and often the answers to your burning design problems are not easily found. By keeping your work, you create frames of reference or reference points that allow you to see where you have come from, your previous mistakes, and how much more you will need to do."
Admittedly, I have trouble throwing ANYTHING away (wanna read a two-year old copy of Wired magazine? I'll send it to you as soon as I get around to reading it myself...). But in the case of your ideas, old outlines, partial drafts, and anything else relating to your writing, I think it's justififed to keep your work. It's not necessarily that you will ever turn all those into finished projects, but they can definitely enrich the projects that you DO choose to do.
One good way to keep all that stuff is to get a big notebook and write in it directly but also Scotch tape into it any scraps of paper, napkins, or anything else on which you jotted down an idea. For longer works (e.g., half-finished short stories or novels), give them a separate folder but jot down a summary in the big notebook.