It is an acquired skill. Two points are most important:
1. You know the value to you of what it is that you want. A satisfactory deal is when the price fits that value. That an even lower price might be possible, or that someone else might get a better deal, does not matter..
2. You have to be willing to walk away when you can't get the price that matches value. Otherwise, you are at the mercy of the negotiator on the other side of the deal.
I've been in negotiations for assets at the billion dollar level. If we can't figure out #1, we had no business being there. That can involve a lot of homework, a significant investment of time (andtime is money). I've been in negotiations where my management didn't pay attention to #2, had to win at any cost, and sometimes winning meant losing.