E.1] Piety, a.k.a. "We'll need priests. Lots of them."
Piety is fairly easy to come by. First of all, any and all generals start with 3/10 piety barring any bad trait like Superstitious, Feck or Public Atheism, and there are five main ways you can get more : crusading, ReligiousActivity, PublicFaith, ForcedReligious and TouchedByTheGods.
Crusading is fairly straightforward : answer the call, ship off to whatever den of cursed heretics has earned the wrath of the man in the funny hat and hopefully conquer it will get you the Grand Crusader trait, for a whopping 3 Piety and assorted Chivalry. Muslim factions have an equivalent trait for successful Jihad leaders.
PublicFaith has 4 levels, each giving one point in Piety. It can be gained by sitting around in a town that has a religious building (15% chance for 1 point every turn), by having a father with the trait (20% for 1 point) , by being born with a cathedral or better (or jama or better) anywhere in your empire, and lastly simply by being born (4% flat, with increased chances if you're playing as HRE, Spain, Milan or Poland, and always one point for all Muslims). This cannot really be trained, you'll have to rely on that 15% chance each turn. Expect 2 or 3 points from that over a governor's lifespan.
ReligiousActivity also has 4 levels, and is gained through building all kinds of churches as well as training priests, one point for each. Note that you'll also get chivalry by building churches. The max level is reached with 12 points, meaning you can get there by building 4 small chapels, training the 4 priests, then upgrading all 4 to small church. You can also build a small chapel, destroy it then rebuild it on next turn but beware if you're playing a catholic faction, as the pope is not too keen on destroyed churches.
ForcedReligious you get by getting tried for heresy and surviving. A general can also get it randomly by surviving a disaster, and a bug in the game somehow makes generals check that trigger each and every turn, albeit it's a very low chance (5% of 4% IIRC). Sadly, the same trigger can also make them Superstitious, which is bad, and can pass down to their offsprings, which is why you want the Rational trait. All in all, this trait cannot really be trained, but welcome the sight of an inquisitor in your region : in 1.2 they very, very rarely succeed their trials, so if one gets close to your governor, you can gamble on it and evict every other possible target for the inqui and hope for the best.
Lastly, you can only get TouchedByTheGods at birth, but not only is it a very good trait (giving +2 to +4 Piety), it's also self propagating, meaning it gets better all by itself over time. The bad news is that having it means you can get neither Rational nor Sane, and you could very well wind up with a Superstitious or even mad governor.
EDIT : pete101 has found out that the King's piety influences the income of EVERY city/castle you own. Send him on a crusade already !