Do I just have to keep winning and keep all my vassals in the dungeon?
It's very realistic part of the game, really - you should to be stronger that at least third of your vassals together. It's not strict math, just my observation.
Upgrading demense and rising retinues is VERY helpful. If you're managed to vassalize holy order or mercenary pack, it's useful to have them rised - they don't count as your troops when they are in your troops tab, and they can be vital in troops count when faction strenght calculating.
Also it can be useful to max fragmentation in your vassal realms before granting duchy/king title - new vassal will be weaker and giving you less benefits, but he will take his time trying to preserve order in his own demense (and you should know already - that's not so easy

).
Also you really want to have as much vassals as possible to your limit. If you have limit in 25, and can create 3 dukes or 2 dukes and couple of counts without going out from limit, take latter.
Also it's useful to have good Diplomacy and restrain yourself from tyranny options until you REALLY need some. BUT it's also useful to remember that it's simpler to win when you have anti-tyrany rebellion that faction one.
Also it's useful to have Imperial Administration - not just because it grants duchy viceroyalty with free revocation of such duchy, but also because it's allows you to use Retract Vassalage option without tyranny. So it allows you to play such plans as "give a troublemaker Adviser place in council, strip him from vassals, allow him to rebel, crush him, remove titles, repeat with another councilor".
It works for me.