There are two rules for negotiating that are absolutely essential.

1/ Be able to make the other side's argument as effective or (ideally) more effective than they can. If you can't do this, then you’re lacking empathy for the other side which is very dangerous. If you can’t do this, then you likely don't truly understand the issues at hand, which means you won't be as effective as you could be. Never negotiate without doing this.

2/ Internalize your BATNA (best alternative to a negotiated agreement). A lot of people know what this is, but very few do it, often because they don't even want to consider the idea that they won't get a deal done. You have to go there. If you don't play out the worst-case scenario in your mind, internalize it, and understand that if you can’t get a deal done the sun will still come up in the morning, you are in a much weaker place. Get comfortable with your walk-away position.

These are two simple things that very few people do consistently. Following these rules will make you much more effective at getting to a deal that works well for both sides.