Apart from factions, we should also cover retinues.
For general info, go to
CK2 wiki. What we need to say here is that as long as you have a large retinue which together with your demesne levies outnumbers the levies of your most powerful vassals (it's possible for a king to have a smaller levy than a powerful duke vassal), especially multiple times, you're relatively safe. You aren't necessarily safe from losing fringe counties to your enemies but you're safe from being toppled. I'm not sure but your retinues should count as "top liege current levy", which means they could in themselves be a deterrent against faction rebellions.
I don't use retinues to replace levies and get even: I believe combining them with your demesne levies for an overpowering force is the better solution. Conserve your troops, secure numerical advantage for battles and especially for sieges, where overpowering numbers are at their best. But, as far as sieges go, you don't want to use cultural knight retinues and other expensive retinues if you can avoid it. Assaults will bloody them, so will disease and other events, that can run you up in debt a lot.
This said, sometimes you need to use knights in sieges. They would appear to be useless intuitively, but they still count as troops and contribute to lowering the enemy morale towards ending the siege facter. It's similar in assaults. And sometimes you need to assault. At some other times, assaulting simply pays better. If you have something like 12:1 numerical advantage, it doesn't really pay to wait for a series of events where you take losses (your own events or events spawning for the enemy). Basically, assaulting reduces the chance factor: you take losses but you know more or less how many. Additionally, if you're suffering attrition, you generally just need to assault because when you're assaulting the attrition isn't hitting (just like in battle). This means that if you hypothetically timed the assaults appropriately, you could avoid attrition for several months.
However, if you rely on assaults, be prepared for the enemy dragging it on. In such situations, surrenders around 30-40% are unlikely. These days the AI tends to resist all the way to 100% if you quickly take its holdings via assaults, unlike some situations I remember from the past patches with surrenders around 30-40%, sometimes under 30%. Those are more likely to happen in more traditional conflicts (slow sieges, battles maybe). On the other hand, the AI isn't allowed to continue the war if you do hit the 100% with assaults, so make use of it when you have to.
Using these general guidelines, I've been able to survive successions with underaged kids succeeding to like six kingdoms, including land on islands and overseas in Palestine. It was difficult and sometimes the gifts could cost up to a thousand but it paid back (war costs more). I didn't lose land in those successions and I believe they were generally without issues. Few rebellions whatsoever. The faction system is a very useful tool for a ruler to keep his realm at peace with this mindset.
If you're interested in details of conducting powerful expeditions against rebelling vassals or enemies attacking your revolters, go to my blog for a post about
Sieges and Seas. It covers these things more in depth, and the information is useful in dealing with typical Big Bads like the Fatties or Rum and, in some cases, the Mongols, which includes guys who are more powerful than you are, on paper.
Speaking of which: a count with a single well upgraded province can outmerc a king and win. A Christian defender with ~500 piety can hire ~22.5K in holy order troops without paying a penny (just be careful because provinces in the Holy Land rarely offer such a high supply limit and you can't divide orders, each order is its own unit ~7700 large). I went and defeated an Ilkhanate invasion of Jerusalem against my king once when I played as a single-province vassal (for the record, the battles brought back much of the piety, so you aren't really losing all of it). So: remember to make use of what you have. And look for things you can use. Stay on target, don't waste it on things that don't matter, go for the war goals and all, but prevent the enemy from becoming able to counter your advantage (you do want to break an almost-finished siege if you run the risk of two small enemy armies combining into an army of your own size or larger, don't be greedy). Rationalise. Allow yourself some save-scumming perhaps, to reload after bad moves or incidents of inattention leading to bad consequences. Replaying the same situations until you can defeat them helps you learn (as does dealing with the bad consequences.) As a Christian remember about piety. Being virtuous not only runs up your piety but it also makes your vassals and other people like you more. Being Kind, Charitable and Brave at the same time really makes a difference. On the other hand, negative traits hurt your relationships and thus indirectly make rebellions more likely.
And, once again, remember about your economy. Wars aren't really won on the battlefield, according to some people. I'm not sure I agree but there is much truth in that.
One more caveat: As a patrician you need to make your floating reserves larger. It's possible to hit several events for -50 gold in a row. When your income is like 8 gold monthly and you have a bunch of mercs hired in a war against a bid bad emir, that hurts.