This is the most important aspect of negotiation. You must convey to the salesperson that you are perfectly willing not to purchase, and you will walk out unless you get a good deal. 

Without this, any decent negotiator will hold his position, because it’s clear you are going to buy anyway. In the car business, if the frustrated buyer storms out of the sales office to get in his car and go home, the salesman will chase him down. 

I even had one salesman step between my car and me to keep me from opening the door. I had to either laugh or choke him out. 

Either way, the deal always gets better at that point. If they let you go, the deal was as good as it was going to get that day. 

Either way, never get emotionally attached to a potential purchase. Even if you do, don’t show it.