howandwhy99
Adventurer
The default as I understand it is:2. Reward roleplaying. Make sure that the benefits from diplomacy far outweigh what they'd get from killing someone and taking their stuff.
A. XP for killing a monster + GP cost of treasure balanced by worth of treasure to your class.
or
B. An ally who increases your group's level and abilities available as well as all the treasure.
I believe D&D is set up to lean towards cooperation versus competition. This is part of why 1-on-1 solo games were considered different to team games where each player was constantly assessing how they aligned with everyone else.
As far as advice, check out the game Diplomacy for how instill political interactions between players. Politics begins at home and in D&D one of the first and most common issues is Treasure Distribution. It could be such a issue that the books often included advice for settling matters before they could become contentions between players. But I would disagree with doing that. Those are not rules and how a party purports itself is a huge part of the game as a group activity. At least hold off on such heavy handedness until each person is put in the position of understanding how gaining treasure can be a benefit to them.
Engaging with NPCs politically, meaning governance and their perceived authority in relation to you, can be part of every NPC encounter. Tell the ogres you were sent by their clan chief lead them. That'll take some doing. But it changes the dynamic of the game by entering into other content and game structures. All of which is what you want to do. So, first and foremost, implement that content and structure into your play material. Once the NPCs are behaving as more than simply combat targets the players should twig to the idea that the world has more going on than your standard MMO.