What makes some people better at negotiation than others?

I believe good negotiators are well prepared on details and always know what is attainable.

Additionally to the previous comments, i consider that emotional intelligence is of paramount importance when it comes to negotiation.
Following this the good negotiator negotiates over interests and not over positions.

Negotiations is a key factor in business transactions you must be with skills plus flexibility emotional intelligence as mentioned by my colleague is a must this it makes some people to be with good abilities via negotiations.

Experience and learning skill development.
Negotiation is not something you born with, it's a skill that you develop through learning and practice.

The Bargaining Power the Organization or the individual holds may be a factor in getting an advantage over others in terms of Negotiations.

It all depends, how smart are you, while bargaining, you have to make sure, you buy/sell, when you negotiate, you earn, not loose anything, first a good market survey should be done, before going for a negotiation on a final deal, not every deal is a final deal, in regards to have reputation of being a good buyer/seller.

preperation,listening skills,experience,investigative work pre to negotiation.

always be prepared,have all the info.

know where you stand before you go to the negotiation.

 

Every negotiation has an outcome, people who knows how to foresee these outcomes by using various leverages gets the kill.
It's even more interesting when two equally good negotiators negotiate, when both are looking for the same outcome.