about the pope: gift him cash, send priests to whatever territory that isn't of your faith to get a load of cardinals. I mostly played Denmark, and sending priests into Russian territory was a cardinal magnet.

All other factions: I just gift them when rep gets low.

But with that said, diplomacy really is a minor factor in this game IMHO. You might as well give everyone the finger and go pillaging, it's not like you'll have much resistance anyway.