My uncle used to work a dealership lot and he always told me the most effective negotiating tactic was to just walk away. Works like a charm, especially when sales are slow because the manager is going to tear his sales staff a new asshole every single day they haven't moved inventory. This was how he got my dad a few extra grand off an Infiniti, he was with him when he was buying it and he convinced my dad to walk away even though he was ready to sign. Home landline was literally ringing as he walked through the door with the dealership on the other end begging him to come back.