Althought Franco is off already, here is a trick that I have used.
I find that booking directly with Alamo or National is a cheap as anyone. So I book the best rate I can find THEN as I get closer I keep checking the site, ESPECIALLY the last minute deals.
Two weeks ago I went from $35 per day for a full sized car to $20 per day on a last minute deal. Booked the $20 price then went back online and cancelled the $35 rental.