I would say preparation is the single greatest factor you can have to win a PK fight. To me, this includes planning beforehand what skills/spells you are going to use in fight as well as putting up your defensive abilities and whatnot. If you have time to prepare, and your opponent does not, the fight should not even be close. If you both have time to prepare, I think it's simply a matter of who prepared better. I suppose I'm talking about roughly level 40 characters here, because things are different at the low ranks, where sometimes no amount of preparation will win you the fight.