So what's your experience/tactics for succession?
PAUSE THE GAME IMMEDIATELY.
Usually, what I do is if my old ruler is nearing old age, I tend to stockpile cash. Why? Even if the heir has some cash himself, a little more won't hurt. I use this money to send as gifts to uppity vassals
and family members who hate the new ruler (because of claims).
Next, I check to see if my council is still intact - if the new ruler was previously already a ruler of some other place (for instance, sometimes I make my heirs a count of someplace if I'm a duke or king so they can "gain" "experience"), then their council will sometimes be
their old council, not the old ruler's. I make the necessary switches, of course.
Finally, another preventive measure i usually do in the previous ruler's reign - I make my heir the mentor of all his siblings. Doing this ensures his siblings have a +25 relations boost with him (for being their mentor) and is a bit of insurance against some nice sibling rivalry in the future - it doesn't guarantee it won't happen, but it helps. What I usually do is mentor the other kids myself until a month or so before they turn 16, then give them over to my heir - that way, I can still choose the kids traits (instead of the AI which is kind of random), but the heir is technically still the kids' mentor and the heir usually has my career trait or whatever so it works out fine.
Oh, and panic, yeah, that's a good tip.