In fact, there IS an article about how to play/beat Possession. It can be found here:
http://dominionstrategy.com/2010/12/03/alchemy-possession/But basically, Possession is usually not a winning strategy; it's just too cost prohibitive. Every time you buy Possession, you missed an opportunity to buy a Province. Dominion is often a fast game, and there isn't necessarily time to go for this expensive Potion card if you are only going to be able to use it effectively once or twice.
In general, a good way to beat an opponent going for Possession is to junk your deck. Start loading up on Duchies and Estates earlier than normal, and by the time your opponent gets around to Possessing you, they want have much to gain. And remember, you gain the VP chips, so cards like Monument and Bishop he can't even really play when they are in your hand. Same goes for other attacks: your opponent can't really play your Torturer or Mountebank, so that end sup being a dead card for him (but not for you).
By the same token, watch out for atypical situations that make Possession better. An engine that can consistently play multiple Possessions (possibly with Hunting Parties or Golems or Kings Courts) can make it a lot more worthwhile.
I can't see the game you played, so it's hard to know if your opponent got lucky or what. I would think a Wharf/Money strategy would be too quick for Possession in a Province game. Perhaps you should have loaded up on Duchies earlier. Wharf often gives you $10 with two buys, which would work perfectly here.
