I think the overall keys to possession is, you want your deck to be crappy, or unable to be used by your opponent, but still able to play possession often. You want your opponent's deck to be nicer than yours, and then use their deck more than they use yours (or even your own if you can manage it). Add to that the volume of different asymmetric interactions due to the special-ness of possession (for example, the possessed player gets the +VPs but the possessing player can use up their coin tokens) to make your deck less useful to your opponent.
Every other detail is some variation of this (for example your opponent can't really play +VP cards when posessing you, so if you have those they become one-way dead cards: useful for you, a dead card for your opponent, coin tokens are a liability since your opponent will spend them all if they get a chance, then there are a whole host of cards with asymmetric benefits when posessed: hoard is useful mostly for you, since if your opponent is smart they won't want to load up on golds in their deck, same idea with golem, for attack cards you can be more possession-immune if you have cards like ghost ship which slow sifting down, and/or junking cards can stop a possession strategy by increasing the time between possession plays (though in general you shouldn't go for both junking cards and possession since you will be the one suffering from the junk), it may be useful to get your opponent to possess yourself to get rid of a bad hand so you can get a shuffle faster etc, it's great to have a deck with nothing but sifters and possessions etc.)