By understanding the mechanism of it and making the imprint so strong with divine intervention that it never changes? Woedica apparently has his soul well in hand for every part of the cycle. They even manage to tweak his biology so he's always born exactly the same man, physically, too. He's not just thaos' soul in a bunch of different bodies through the years. He's always the same guy that originally was born in Engwith. To Thaos, at least, the biology does seem to matter.
EDIT: Wait are you talking about his soul projection when he possesses people? That's always very short term, and still using Woedica's cheats and Thaos' knowledge of animancy. Plus it's usually cohabitating with another soul. And finally, I never denied that souls retain a memory of what was there before. I'm seriously not sure either of us understands what the other is getting at.
Edited by The Sharmat, 06 April 2018 - 07:34 PM.