You have already lost if…
You aren't willing to walk away. In every deal you look to do during negotiations, if you are 200% married to the outcome as you see it, you lack leverage and are more likely to make poorer decisions. You can't negotiate very well if you don't care what the terms are so long as you have what you want.
If you're making a bid on a house and you know it's only worth $200k, but you're so enamored by it that you're willing to bid $400k in a fierce negotiation with another potential buyer, you've lost. They have far more emotional control than you in that situation.
If you're in a bad relationship and no matter how he treats you, you won't leave, you've lost. And he knows it.
If you're at a start-up and there is no vision, no leadership and despite an ok product, you're just spinning your wheels and no one will call it for what it is (aka a good time to walk away), you've lost.
The ability to see things for what they are and be willing to throw in your hand is just as critical as sticking it out. Again, if you aren't willing to walk away, then negotiating is really just for show sake, isn't it?