Giving a regular tribute to the Pope is great advice to stay on his good side. I played as France and the Holy Roman Empire (HRE) was a real pain in my side. I gave the Pope tribute to raise my standing and then positioned my troops in the HRE lands. Once they attacked, I counter attacked and took a few of there territories before the Pope gave the "knock it off" call. Repeat for a few cycles and they were wiped out. Repeat that pattern for the rest of your enemies.Originally posted by: kedlav
Always keep a diplomat in Rome to give the Pope some cash to raise the Pope-o-Meter, as well as follow the outlines in Zenoth's post. Other factions are significantly less likely to attack if the Pope-o-Meter is sky high.
Another thing that helps is to keep strong garrisons. The AI identifies weakly held neighboring territories and will target them, especially so if they're really close (like the Norman coast near France that Britain starts with). A strong garrison deters the AI and means they'll focus on another weaker neighbor.
Allies don't stay benevolent towards you indefinitely. You must constantly court them, so keep a diplomat handy and keep a close eye on their status towards you. If it starts getting low, do something to appease them. Early on, a map ought to be sufficient. Later, it may take upwards of 50000 florins to move the friendliness meter. The AI also won't like it if you attack someone their friendly towards, if you keep a large army in a bordering province, etc.
Despite all this, AI's will attack sometimes. You may be the easiest nut for them to crack, or the AI may flip into idiot mode and come with a four unit stack when you're sporting two full stacks near the border. Most of the time, a quick strike that takes 1-2 regions and the return of 1 of them will buy peace, so if you've strong border province garrisons, don't be above stripping them to make a lightning strike when a previously friendly neighbor gets uppity.