In Pathfinder and D&D, the Diplomacy skill is almost a joke. The immutable DCs presented with the skill are clearly bunk; they are high enough so that in the first few levels it’s practically impossible for players to affect an NPC, yet scale slowly enough that with a little bit of investment in later levels players can easily convince the biggest, baddest villains in the game to be their best friends. Worse yet, there’s very little guidance on what the levels of hostility or friendship actually mean, making the skill seem like magical mind control as a default. The whole system is a complex thing that doesn’t appear anywhere else in the game. Here are the best ideas I’ve been able to find out there on the wild frontier of the internet about changing the Diplomacy rule:
Justin Alexander gives an incredibly thorough analysis of the problems with Diplomacy here
Rich Burlew, creator of Order of the Stick, has this somewhat over-complex but solid idea, which has become quite well-known
An excellent rule idea called Social Combat, which makes some major changes to the way roleplaying skills work, can be found here.
Alternatively, one of the simplest things you can do is to make Diplomacy opposed to some other skill, such as another character’s Diplomacy or Sense Motive roll. Have players make checks every time they want to convince someone to do something (just like how it works with Bluff), and you’ll solve many of the inherent issues with the skill.