For my part I use to send a diplomat to each and every faction on the map and rotate them as much as possible (one chasing the other) and negotiating map info or offering to give a region every turn (which I rarely do because the AI refuses almost every time)

Playing the Long Road mod, that means around 20 diplomats on the filed at any given time and 20 negotiations every turn... All of my diplomats have around +5 and more...