Find a common enemy.
If you can't find one, make one up.
Human beings are social creatures and as a result, there is an innate human desire to be part of a group, to be part of a common cause working together towards a common goal. It's the whole concept of us vs. them - whomever "them" might be. You can take advantage of this part of human nature during negotiations by setting up a common enemy - someone that can be framed as even plotting against both you and the person on the other side of the negotiation. It may seem odd that I would actually advocate aligning yourself with the other negotiating party, but there are times where you can do this to great effect.
A very common tactic I would use during negotiations is to align myself with the salesperson against someone else who needed to be overcome in order to close the deal - the common enemy. Often, this someone else was my wife.
I would often tell the salesperson things like “Look man I really like your product. I have been talking to my wife about how much we need it, but she remains unconvinced. She keeps on harping about how much it costs. I think I have an opportunity to get her to say yes, but you gotta help me out here. The next time I talk to her about this, you gotta give me something that I can talk to her about to assuage her concerns about the cost. Help me help YOU make this sale. Please help me help YOU.”
I will say that I have never run across a salesperson where this tactic did not work. Most salespeople are geared to view sales transactions as border line adversarial (the whole Glengarry Glen Ross “Always Be Closing” mantra). It throws them for a loop when they have someone on the other side of the transaction that seems to be on their side and is even openly advocating for them.
It's important to remember that the salesperson should never actually meet this other person that seems aligned against them from making the sale. In order to keep up the ruse, this other person should remain some mystery figure who is actively plotting against them. Kind of like an evil James Bond villain - that way there is a sense of subterfuge and scheming going on which adds an element of thrill to the transaction. And with that element of thrill, the salesperson will start getting creative in terms of concessions - I can't tell you how many times a salesperson will say things to me like “How about if we do this - do you think your wife would go for that? We need to figure out some way to close this deal!”
Just make sure the other person is plausible - that's why wife / fiancée / girlfriend (or husband / fiancée / boyfriend) is always a good option in a personal transaction. Every once in a while, I'll even have my wife “walk in” on me when I’m on the phone with the salesperson and have her rant and rave about something (it doesn't even have to be about the subject of negotiation - she actually uses this as a stress reliever). When she walks out of the room, I'll then whisper to the salesperson “Dude you gotta help me out.” Sympathy on top of subterfuge tends to be a winning combination. In a business transaction, I always just blamed my “boss” or the “bean counters in accounting” - somebody anyone can relate to.
My wife has even used this tactic quite a bit with her own negotiations - blaming me for all kinds of obstacles in her purchasing life. I am sure that from the outside looking in people must think we have a really dysfunctional relationship as it appears that we are always scheming against each other, but it's all in the pursuit of a better deal. Of course, if I ever see my wife stroking a bald cat and / or start looking at maps of the Pacific Ocean looking for deserted islands, I'm going to start thinking something might be up.