Tip: Exploit of "Request to War"

Whenever you don't feel the need for expansion (to delay the Realm Divide), why not extorting some koku from AIs?
Requirement:

(1) An enemy that is badly beaten and is willing to pay for peace. If it is not, try to march anything into their territory. Suddenly it is very willing to pay for peace.
(2) An ally that is warring that particular enemy.

What to do:

(1) Accept the enemy's peace treaty, and ask it to break all other alliances (so you don't complicate things when you declare war on them later). Take their koku.
(2) Go to your ally. Use "Request to War" and offer your service declaring war to the same enemy. Your ally is likely to pay for your effort, too.

So you can simply repeat above and extract koky from both your enemy and your ally, as long as your enemy remains weak and is willing to pay for peace. There is no penalty for breaking the peace treaty. Nice!