4. DONT LOSE A CRUSADE!
There´s a nasty experience I made here: If target province of a crusade is taken by another Catholic faction the crusae is considered as lost as well! Mine (HRE, what else ;) ) was still miles away from its destination (as I assembled it in Swabia - my provinces at the time had sky-high zeal, so that made sense, with gathering up new units on the way) or any fighting, but the sneaky English took the target province, my crusade was disbanded and half of it decided to join the rebels.