Save old road tires and cut them into 2" strips, entirely across the tire (or if the wire bead's too tough for what you have, use scissors and punch through the tire beside the wire, cut alongside it, across, along the other side, and back) and keep a couple in your seatbag. They beat the hell out of dollar bills for covering big slashes in your tire, ones that would otherwise let even a heavily-patched tube extrude and flat. Resist the urge to duct-tape or electrical-tape them into the tire casing and keep riding them once you get home: the adhesives in many tapes slowly attack tubes and leave them riddled with holes. Fiber-reinforced packing tape seems to be an exception to this rule, which is why you can use it in place of a rimstrip if you're missing one during a wheel build. They eventually come apart, but it takes months, so you have plenty of time to find a rimstrip at your LBS.