The standard answer is practice, practice, practice. Repetition; however, is the mother of skill, but what you are practicing is just as important. If you want to be better at basketball, you aren't doing yourself many favors on the golf course. If you want to be better at negotiation, you aren't doing yourself much good reverting to bad communication habits like "tell me more" and trying to convince the other side by exposing them to your logic.
Remember negotiation is an information gathering endeavor based on good communication. How in fact do you stay good at communication? Not only by having conversations but by consistently putting people in a place where they trust you enough to think out loud in front of you. Make it a habit in every conversation to make people feel so comfortable talking to you that they tell you things they normally wouldn't tell people.
A good friend of the company, who not so long ago was just an avid reader of “Never Split the Difference,” said one of the most amazing things about how to approach negotiation I've ever heard. He said “now that I have read the book I no longer try to convince people of things, try to get them on my side or get them to understand my point of view. Instead, I focus only on how I make them feel.” It seems so simple, but it was one of those things that after a bit of a discussion I understood to be simply brilliant. For example: if someone that works for him makes a mistake he knows he is wasting his breath my explaining to them what they did wrong and how to fix it in the future. Instead, he communicates with them in a way where they genuinely feel like they should have done better and have a sincere interest in improving and never making the same mistake again. If he is negotiating a real estate deal and trying to get a lower sale price, he doesn't explain that they can't sell to him at a number for various reasons. He makes them feel like if they want to cut the deal, they will have to come to his terms. All human decision-making is based out of fear or love. He uses this concept to trigger an emotion that will guide a person's decision making. If he's trying to get people to consider other data in their decision, he'll ask "How does (additional data) affect your (current choice)? This puts people in a place where they have fear about having not considered something and making the wrong choice, or out of love, they begin to think about what more they achieve if moved in a new direction. He does all of this by using skills he learned in the book, and consequently, he has gotten very good at Tactical Empathy. By consciously changing the desired outcome of every single interaction he has, his communication skills have skyrocketed.
A phrase I have stolen from my father Chris, which I am sure he stole from somewhere else, is “it is amazing what people will do for you if they feel like it.” This isn't that far from “never be mean to someone that can hurt you by doing nothing.” I was returning a rental vehicle once and as I pulled the car up to the return line the gentlemen checking the cars in literally says to the woman in front of me “people will do amazing things for you if they want to.” I don't know the circumstance or the context because that was all I heard, but the lady did look a little upset, and his tone sounded like he was taking an opportunity to teach her a lesson. If a service guy is saying that to a random disgruntled customer, it has to be true right?
One of the great fringe benefits of being a negotiation consultant is the constant study of human nature response, despite culture or upbringing. Just being acutely aware of how people generally respond to things in all situations. If it rains people go inside if it is sunny people go out. If someone has an idea, they instantly become dedicated to it in some fashion. If it is your idea, others have questions first. In negotiation, the problem is when we achieve agreement because we put pressure someone. The desired result is obtained but the residual effects of noncompliance or poor follow through are never acknowledged, and we wonder why things fall apart. As human beings, we generally do not learn from success. In is the same reason the human culture is a reactive one instead of preventative. We add more security because people got shot or blown up, we create the stop sign because of so many car crashes.
Maybe our friends objective in a conversation doesn't work for you, and that's fine. The point is, he expanded his negotiation ability by changing his outlook to a given series of situations. If you are trying to improve at something you need to challenge yourself. Staying at the same level doesn't make you better, you plateau. In fact, in those moments you actively engage in insanity - doing the same thing over and over but expecting a different result. If you want to get stronger and lift the same amount of weight every day, your muscles will never grow. You need to change the workout routine or set higher goals. The answer to building your skill set is making yourself uncomfortable and accepting you will say the wrong thing sometimes. Complexity is not the barrier to learning; it is in fact awkwardness. Challenge yourself by changing your aiming point in negotiation. Instead of trying to get a “yes,” make the other side say “that's right.” Don't try to close the deal in one conversation, as an alternative put them in a place where they want to tell you things they are playing close to the vest. The bottom line: you will have to change what you say to people. This is easier said than done.