Harvard Business School professor Deepak Malhotra offers 15 pieces of advice on how to negotiate your salary or job offer. The first component, he says, in the negotiation equation is: "They need to like you."
By getting the other side to like you, they'll be more likely to want to help you, another critical component of the process. This means, as Eric Barker points out on Barking Up the Wrong Tree, that the negotiation strategies you use at the table matter much less than what you do leading up to it. From the video (about 2 minutes 45 seconds in):
A lot of the action that happens in the job market season isn't necessarily at the table. It's not when you're sitting across the table from the person you're negotiating with. It's a lot of the stuff that happens before or after.
It's not just about merit or your strong arguments, although those matter too. Just don't forget the emotional component.
The video above is pretty long, but it's full of excellent negotiation advice.