A good number of receipts are printed on thermal paper these days. Look at them after a few weeks in your wallet or other warm spots, and they're unreadable. A Consumerist comment thread picks up a few ideas on keeping them useful.
Photo by receipt on Flickr - Photo Sharing!.
You can't do all that much to prevent the wearing away of loose ink on cheap paper, but you can ensure that you get the tax credit, expense reimbursement or item return you want by locking in their verification. One way of doing that is initialling the receipt with a permanent marker before it fades and photocopying it, so the photocopy carries a bit of authenticity. Other ideas brought up at The Consumerist blog:
Scan a month's worth, zip the file, and email it to yourself. Also, burn a copy each month and store it with your receipts.
... "Photocopy, and staple both the original and copy to the inside cover of the manual. If the original still fades, you have the copy right there.
Meticulous, as-they-come-in scanning is always a smart move. If you know of another receipt-saving means or magic, tell us in the comments.
How To Protect Your Receipts [The Consumerist]