I am still looking into this but for now here are the best practices I have:
* ALWAYS be the strongest in the realm (don't be like King of France: 'King of Paris-only') in terms of highest number of demesne (good for depth in case of an extended war), highest income/troops (highest # of castle type baronies)
* Try and grant land to people who have Content (+50) trait // Do not grant land to people with Ambitious trait as well as others (possibly envious etc. can't remember which ones now - check the excellent traits spreadsheet)
* Try and keep your demesne together, under two duchies so that if it comes to worst, and you fight all vassals at once, you can merge your armies together before attacking/defending (if you have 1 vassal between each of your holdings, difficult to defend)
* Definetely a good idea: keep a war chest (I start with 200 gold, increase to 1k, 2k, 5k as time goes) this is not to be spent on anything but kept it treasury at all times, so even if UFOs land on your all holdings at the same minute, you can still fight back effectively using a concentrated force of mercenaries
* Potentially a good idea: keep a single far-away county (perhaps of city type-- need to test this and see if rebellions will be manageable due to no-border issue) so that none can easily conquer your all provinces at the same time (thus war does not auto-end). provided that you've kept war chest full per above, you will have time to fight back when your enemy sails to your far away county/barony.
* Land distant relatives as they have +5 relation however they should not be 'pretender' or close relative -- still testing this
* Keep two layer of demesne,
the inner demesne: 6 baronies (6 castles) this is the fallback level when/if your underaged heir inherits. This must be the strongest/fully upgraded counties, ideally with 3 castles or more on each, located next to each other
the outer demesne: 6-7 baronies (one or two could be city) the 'claim back' level when your underage heir is crowned you distribute these to NEW lords (they have single county each) and once your heir reaches 13 county level again when he is 16 and marries a high steward lady (so that he does not have penalty anymore). try and obtain these back asap.