If it's under 75$, then you don't need a receipt. Just a "record" of your expense (could be as simple as a note in your logbook)
Should keep them for 3 years from the date of filing....so ball park, you can throw any receipts earlier than 2005.
(one caveat, in the event of fraud, there is no statute of limitations for when the IRS can nail you for taxes and penalties)
http://www.irs.gov/pub/irs-pdf/p463.pdf