I use (and recommend you use) FreshBooks for invoicing and tracking of expenses at Lemon Productions and they just added the ability to import PDFs as a receipt. So scan a copy of your receipt to PDF and attach it to your expense report and then never worry about having to store and find a paper copy of your receipts again.

Video Version

As an added bonus, keep a copy of all your scanned or photographed receipts in a folder in your free Dropbox account and you'll have a backup stored in the cloud in case your computer ever disappears.

Even better, if you take photos of your receipts with a free Evernote account on your iPhone and you'll be able to easily search the text of your receipts later thanks to Evernote OCR'ing (recognizing the text in your images) your receipt.

Like magic.