When I bought my last used vehicle (from a reputable national manufacturer dealer, not a corner lot), the salesmen weren't allowed to negotiate numbers at all, it was all done by the finance manager. And his starting position was "Used cars are already priced at the lowest possible price so we can move them off the lot quickly." He absolutely refused to lower the price of the truck, but after I literally got up to leave, suddenly he wanted to take another look at my trade-in, and it magically went up in value 2K.
Key is to stick to your guns. You havea budget in mind for a maximum monthly payment AND total cost to you (please don't buy purely on payment amount...), and you need to not budge from that maximum.