Flinching
Some people think that flinching is a sign of weakness, but in the world of negotiations, this is actually a great tactic to use. Whenever you are buying anything or negotiating a price on something with someone else, try flinching when they bring up their offer, you may just save some money.
Just flinch slightly, nothing over the top, but enough to see that they know you are not satisfied with that number. They will usually panic and try to immediately lower the price or negotiate even further with you, which is exactly what you want. Who said flinching is for the weak?