A month or so back I wrote about some common PC errors when when negotiating with monsters.
This is a look at how you do it in GURPS Dungeon Fantasy, specifically negotiations with intelligent monsters.
My primary source for this is Dungeon Fantasy 2: Dungeons. GURPS also has Social Engineering, but that's a pretty heavy expansion for a game that's myopically focused on getting loot from dungeons. DF doesn't even reference much of Martial Arts, and combat will outweigh negotiations in any standard DF game run as-written. This post assumes you're familiar with negotiations as written on page 10 of DF2.
First and foremost, negotiating calls for good player skills. You need to be able to understand what's at stake for you and the other side. You need to know the value of what you're trading off in a deal. You need to have some ability to create a reasonable framework for negotiating and settling.
For the most part it ultimately comes down to a skill roll - a silver-tongued player running a character with no Diplomacy skill, a pile of reaction roll penalties, and a Reputation (-2, Lying backstabbing nogoodnik) can't talk his way out of his poor choice of point spending. Even a tongue-tied player can get some placed with a well-crafted Bard. But you can't even get to the roll without role-playing.
Knowing your enemy helps. What do they want?
Nothing torpedoes negotiations like a fundamental misunderstanding of what your opponent wants or what they find valuable. You can ask, but it's generally better to have an idea walking in the door, or you've skipped right to "negotiating from a position of weakness."
Tip: Think it through, but don't overthink it. It's easy to go from "fear of missing out" or "offer you can't refuse" to "offer you don't understand" and "TL;DR."
You also need to know what your potential negotiating partner is like. Beyond detecting trickery, you want to know how they regard deals. You don't want to spend mere seconds agreeing to vague terms when negotiating with a high-IQ foe with Law and a penchant for "technically within the terms of the agreement." O conversely spend hours hammering out precise wording with a low-IQ foe who isn't going to really get what you meant by "shall not directly aid and abet those we may consider foes at this time" and might just think something different than you anyway.
Diplomacy, Fast-Talk, and Intimidation are the main character skills you'll find useful in negotiations.
To sum up what those skills do in a sentence: Diplomacy is the skill of negotiating, Fast-Talk the skill of trickery, and Intimidation the skill of coercion. Diplomacy gets the other side to agree - they might not hold to the deal, but this gets a deal made. Fast-Talk gets the other side to believe some trickery or lies long enough to let you act - usually to get past without a fight (this time) or gain surprise in some fashion. Intimidation gets fear-based cooperation, rather than agreement. Fans of misapplying Machiavelli love to rely on this - better to be feared than loved, eh? But fear of you has a way of weakening as you weaken, and ideally, you want the monsters a lot of afraid and yet very fond of you all the same.
In short, use Fast-Talk for temporary trickery, Intimidation to inspire fear or scare foes or get a combatant to back off, and Diplomacy to get some real lasting agreement. You can use these to help each other, of course - using Fast-Talk to get someone to listen, or Intimidation to get you foes to back off so you can initiate real talks, for example. You can try to use them as Complementary Skills, of course, but they can poison the well with failure. There is nothing wrong with being scary, but it's more useful for short-term effects than long-term ones.
You don't want to be too low skill, either, as there is a default -3 for cross-cultural encounters with Diplomacy. On the upside, though, don't forget that success seals the deal, but failure just means you need to make some more concessions. Only critical failure on Diplomacy causes a true breakdown in negotiations!
Don't fall into the 1-point expert trap ("I'm a negotiator, I have 1 point in a social skill") but equally don't get discouraged. You can always try talking. And a good enough deal might just bypass the roll - the GM might just rule the monsters want that deal and just take it, no matter how poorly you get it across. But if it comes to a roll, realize that like combat skills, social skills work better if you're actually good them.
You need some way to talk - a language in common. The Gift of Tongues spell is a lifesaver, here. Broken language gives a -3, and in my games at least severely limits your ability to impart nuance. (No, "We tell him to do A, or we'll do B, and if C happens we want D instead!" with Broken. You're still at the "This is a pen" level of Goblinese for 1 point.) Accented is only a -1, because you're just missing nuance and annoying each other with poor grammar and word choices. Worse comes to worse, you can try Gesture, but your Diplomacy skill is capped by it!
Besides that, consider Social Chameleon and/or Cultural Adaptability on your Bard. If you don't have a bard, Persuasion spells are helpful to make up for your likely lack of Charisma . . . but it's a resisted spell so it's not foolproof. Serendipity is a potentially good way to find you have something in common, or just happen to have the right kind of loot the monsters want, or something else to grease the gears of friendly dealmaking. Luck might be needed to re-roll your skill rolls!
None of the pieces above really functions in a vacuum. You need the basic abilities to communicate. You need the skills to get a good result. And you need the player skill to offer a deal worth taking to the monsters (and worth paying, as the PCs!) Of course, it's not always going to work - but armed with some of the hints above, it's more likely to work.