I think one of my biggest problems in playing as an assassin is that I keep trying to view GW's aggro system as like WoW's which is completely wrong. From what I've read GW's aggro works logically, targeting the easiest foe first with hopes of taking them out. Makes sense. Now, I was wondering how I would go about dropping aggro as I can't seem to find much on the subject and I find myself tanking from time to time. I would eventually like to drop my reliance on the self-healing shadow arts skills.
There are two guildwiki pages on that, each with sections on breaking aggro:
http://guildwars.wikia.com/wiki/Aggro
http://guildwars.wikia.com/wiki/Guide_to_Aggro