
establish your fighting style-perhaps one of the most critical things you must do before making an assassin, do you want to tank? if so then assassin isnt for you. will you try to take out alot of enemies? try elementalist for that. are you only an assassin for the armor and because the name is sweet? if that is the case please delete your character and do us all a favor.
learn the right tactics-this is probably why everyone hates assassin primaries in cantha more than people hate W/Mo in tyria. as an assassin, the best strategy I find is to go in, kill 1 enemy (preferably one no 1 is attacking and still poses some sort of threat) maybe even 2 (if its not risky) with a combo, then GET OUT!!! most people forget the last part, succumbing to a focused fire due to your attack.
getting away- due to the assassin's...compromised armor shall we say, being in direct mele range is very dangerous. fortunately our friends at ANet have given us an attribute shadow arts, which when used correctly will save your hide. also you need to utilize the no attribute teleports as well. contrary to popular belief, shadow stepping is VERY helpful in PvE. allowing an assassin to get away to relative safety in an instant. unlike a warrior who must amble through his foes to a monk. a good assassin should always be able to get away. almost as important is getting TO your victim so you will need to consider that as well, after all, you're of no use if you cant do your job...kill.
hit and run-my favorite method which i find quite effective is a thing i call "hit and run" basically, you find 1 enemy, get in quickly, kill him fast, and leave before you become the target of a blitz. personally what i do is use recall on someone that wont get into the heat of the fight (like a monk or elementalist or ranger), i then use dash for a quick speed boost while i home in for the kill,i then use unsuspecting strike to deal a great amount of damage (because you should be attacking something at full health becuase you want to drop 1 thing fast to end his rolle in the battle while your allies focus on the others), i then use a offhand to link into the powerful dual attacks, like death blossom, which do 2 attacks that are usually high damaged, the good thing about blossom is that you hit other enemies around you. after you have finished off that enemy, Recall away from the action and heal yourself and energy, let skills recharge (usually you dont have time for a full recharge but if you recharge enough so that you dont have a long wait then thats good too) set up another recall and let your energy buff up so you can make another kill.
dont push it-if you EVER think your life might be comprimised, recall away and heal. DP is HORRIBLE for a sin, as the hp and energy is very important to us and even a measly 15% can comprimise an assassin's abilities. also you will just bring more bad rep to assassins as a class. its better to live to kill another day. however, there is an exception, if its a very critical target such as a monk boss or an enemy that is cuasing hell for your casters, that extra damage you inflict for the sacrifice of your life may just be enough damage for a warrior to finish it off. if you are sure that your death will be worth the enemies death, go for it.
in conclusion, assassins should only be used by players who know what they are doing and know a good bit about Guild Wars. if you are a begining player, i wouldnt recomend an assassin. assassins are a very risky, but extremely deadly class to use. in my opinion, i think that the assassin is the only class that REQUIRES you to have a good technique and operating ability to use. decide your strategy, and utilize it. stay alive, thats what shadow arts is for. and even after a day of no deaths, it's always good to train your technique, so you are better tomorrow. good luck my fellow assassins.

bear in mind im not trying to be a bossy person trying to flame anyone, i want to help YOU the assassin, perhaps if you follow some of my suggestions and gain some sort of a reputation amonst those you have teamed with, you may get accepted more.