My friend knew a guy who basically had standing weekend rentals with Hertz (or someone else with neighborhood locations). Since they don't charge you if you don't pick up the car, he just rented a car for every weekend far enough in advance that it stayed cheap. If he needed a car, he had a reservation...otherwise he just let it go.
I think they might get a little mad about this now, but I've thought about it. There are two hertz neighborhood locations nearby...last weekend I got a car to go to a bike race and ended up paying something like $16 for the whole weekend after I switched my promo code from something that gave me a $25-30 weekend with 2-level upgrade to something that was $16 (basically all taxes and fees) with no upgrades. Didn't matter since they gave me a Rav4 anyways (which was perfect for hauling the bike, and then picking up a bunch of crap in the suburbs on the way back).