This probably isn't the immediate answer you're looking for, but I've found a lot of value in a book called Never Split the Difference by Chris Voss (no affiliation, just loved the book.) It provides practical guidance on the subject of negotiation. However, in reading it you'll find that a lot of these rules of negotiation apply to broader aspects of life. The author, Voss, was a former negotiator for the FBI and was involved in a number of high profile hostage negotiations. (You can't be much more of an asshole than a kidnapper!)

Anyway, he says you can look at most situations as a negotiation. Once you fully understand the needs of your counterpart, you can assess your ability to meet those needs, or work with them (using "tactical empathy" and other strategies) so that both parties understand what the realistic outcomes are. Once both parties truly understand what the other is ultimately capable of providing, they will see no use in demanding more than that.