First things first, do NOT be afraid of tyranny. Dukes and counts will never understand your position, you have to keep order! And sometimes that means stepping on a few toes to do so.

Obviously if you're going to be tyrannical, it helps to have good intrigue to keep the angry idiots from slitting your throat or worse, openly rebelling.

A good marshal helps too, sending him to troublesome vassals you intend to arrest can mean the difference between arresting the stupid sod, or finding yourself in open conflict.

Pagan ruler's have it slightly easier when it comes to other foriegn pagans, so long as you conquered them with the subjegation CB, that's an instant +50 opinion.

EDIT: Looks like PDX nerfed it slightly, it's +75 opinion for 120 months if your religion is the same as the target, and +45 opinion for 60 months if there is a religion difference.
Thanks to Tactical Urban ♥♥♥♥ for notifying me about this patch

Honorary titles! If you plan to clean house with your vassals, give them to council members to keep their loyalty! Remember, an angry spymaster, makes for a VERY dead king.

Vassals are ambitious, they will naturally try to gain as much power within the realm as they can, even the ones with the content trait!

Higher crown authority laws are a MUST! They prevent vassals from waging war with each other and outside the realm. The second point is sort of optional however if you are partiularly large and want your vassals to handle the land grabbing themselves. The benifits of having vassals not attacking each other are pretty self-explainatory, it keeps potential threats from getting bigger within the realm without inheritance.

So you have a problem, one of your dukes has WAY too many counties under his/her rule and has quite a host at their command, possibly enough to tip a faction into the "danger zone". What do you do? Easy, revoke counties. "*Gasp!* But that might make him declare war on you!" I hear you saying. True that they might, but that's where my next point comes in.

Arrest troublesome vassals, a vassal in prision is a vassal that is forced to comply to revokation. Obviously this presents problems, mainly if the vassal has martial skill equivalent to alexander the great, while you have martial comparable to wet tissue paper, this is where your marshal comes in. Send him to suppress revolts in the vassal's capitol! As stated in the first section, this could make the difference between a successful arrest, and a failed one.

Alternatively, have the little braveheart wannabe executed. Sure the new vassal won't be too happy that you've killed his/her relative, but forcing succession in a duchy can potentially upset the balance of power within that title, especially if the inheritor is a child.

Good:
- They make LOTS of money, and their tax is dictated by city tax instead of feudal tax, meaning you could get very rich off their sorry hides.
- They rarely ever inherit titles, instead choosing to conquer them instead with mercs
- Other republics might peg them down with embargo wars, keeping them under control for you
- They prefer to start independance factions instead of claimaint or crown law factions

Bad:
- They make LOTS of money, so they would be able to hire legions of mercinaries against you
- You ALWAYS get an opinion malus for being a different government type, meaning they're far more likely to be a problem down the road
- They have to be on the coast, meaning you might have an angry vassal, whom potentially controls the kingdom's entire coastline.

So you as the king of england decided to conquer Denmark and succeeded. However you have a new problem on your hand, namely the plethora of very angry vassals that don't like you as a foreign conquerer.

Obviously they will try to obtain their independance, you however have other plans. Here's a couple pointers for keeping them to heel and even converting their culture.

1. Take EVERY opportunity to imprision child heirs of foreign vassals. You could then have them educated by diligent members of your culture, which should hopefully turn them to your culture. Best case scenario, your character can educate the heir, making him/her like you as their tutor.

2. Take independance factions made by foreigners as an opportunity if you have the military power, traitors gain a free revokation, one that you can use to take a county away and give it to a loyal courtier of your culture. Make sure to also get that courtier married to ensure he continues his dynasty's reign with your culture.

3. Remember that courtiers are easier to have their religion changed vs provinces, and take far less time to do so. Having foreign vassals of both wrong culture AND religion is a surefire way to breed a hotspot of rebellion.

4. Unless you are trying to make a culture dominant, perhaps try to convert to their culture. Being a character of italian culture ruling a kingdom of predominantly anglo-saxon characters and provinces will do nothing but spell out "I'm an italian with a deathwish"

5. Low crown authority is bad, especially if you have lot's of infidels with titles. You won't be able to freely revoke them otherwise. (Kinda goes without saying, but some people honestly don't know this)

- Try employing a new spymaster outside the realm AFTER incurring tyranny
- If you are confident enough, use faction revolts to get free revokations if you REALLY hate tyranny that much
- If you're facing a potential faction revolt, try as hard as you can to keep them separated. A single rebel stack of 8000 men is far more dangerous than 20 stacks of 400 that you could easily wipe out with a sufficiently large force.
- Like the game says, having vassals with the ambitious or envious traits is just plain stupid, but having characters with both, is the king's worst nightmare. Having those vassals and heirs assassinated before they become problems should be top priority.

I hope this has helped some people better control their vassals, and even forge players who will better utilize politics instead of just slapping it with the sword and hoping for the best.