1) No assassinations(as the AI never does them either)
2) No marriages to old women just for inheritances
3) No grabbing of titles that are not logical: so I won't grab a title of an excommunicated ruler on the other side of the continent
4) No more king titles than necessary: I'll usually grab up to five king titles in a connected area (e.g. Germany, Italy, Burgundy, Lotharingia, Denmark) and any other titles are re-distributed to members of my dynasty.
5) Crusading, not colonizing: I will not keep land in North Africa or the Midde-east unless I move my realm there, rather I will set up independent Christian kingdoms ruled by my dynasty.
And I do occasionally role-play my ruler, even to the detriment of the realm.
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I have recently gained several new provinces that are pagan. How do I "spread" my culture & religion to these provinces?
Spreading religion means converting, which is easiest by event. Keep oppressing them (loot the province or place more power in the clergy), and they will convert to Christianity by event.
Spreading culture means you must be at peace for a while, after some time your culture can spread within your realm through connected provinces. Both the from and to province must be owned by you or your vassal, and you must be at peace as mentioned.
You will not be able to spread your culture across the map, the best I've managed is a few provinces here and there over the course of the game. With extremely few exceptions, the starting cultural setup will be nearly identical to the end's.