The Effective Assassin

Society's balance teeters on a razor's edge. Your judgment must not.
Headmaster Lee of Shing Jea Monastery

An assassin as the name may suggest is a hit-and-run profession as much of the attacks assassins use strike single targets only (with exception to this month's recent update). Thus an assassin is efficient at eliminating targets quickly with a hardy spike. However assassins are glass cannons so they require a careful touch which can be difficult if you are starting out.

Be Aware of the Field
Assassins work well as the bridge between the warriors line-backing in the front of battle and the supporters and casters near the back. Assassins should avoid diving into the thick of battle first. Leave that to the professions with more defense. Upon enemy engagement, some enemies may also charge the casters and suppoters in your team. This would be a time for the assassin to play back up for the casters as they they can't take the abuse considering they are even more squishy than you are.

Watch for your Target
It is generally a good idea to be mindful of the targets your team calls out with any profession. Even more so considering assassins specialize in this. Your best bet is to target the casters when you get the chance as they will suffer the most from your attacks. Even in situations like PvP it is best to go for the casters. Though many will be wise to your game and will throw up their own defences when you strike, it is better to time your attack properly when the enemy least expects you. This can be achieved through attacking a target other than the one you have in mind. (what?) Then when most casters try to defend the one you "appear" to be attacking, simply switch to your real target.

Half and Half
As we all know, you only get 8 skills at any given moment in Guild Wars. With that in mind, it can get very easy to jam your skill bar full of all the pretty attack skills assassins have. A big attack chain is not a good idea though as it is very easy to lose an attack chain for whatever reason, whether it be and interupt, lack of energy, or the target dying mid-chain. A general rule of thumb I follow is 4 or so skills are for striking, and the other 4 are for support or for buffing your attack methods. A good call for attack skills would include skills like Golden Pheonix Strike since it allows you to skip a required lead-attack. Others like Golden Fang Strike are also preferred as it applies Deep Wound to your target which is such a good condition for assassins to take advantage of to begin with since it applies to all creatures unlike bleeding or poison, and deep wound in turn makes any spike more effective.

Order
Anyone unfamiliar with assassins and how their skills work, need to be aware that some skills will require you to connect 1 or 2 hits prior before the skill becomes available. Your skills must go in order, with exception to skills like Falling Spider, Black Spider Strike and the elite Palm Strike.

The order would be:
Lead
Off-Hand
Dual (strikes twice, which means skills like Shattering Assault removes 2 enchantments even though the skill explanation only suggests one.)

Runing Up
Don't bother with major or superior runes as assassins don't have much health to begin with, so just stick to minor ones. As for insignias go, a good call would be Nightstaker which adds armor while you attack, Survivor for the HP, and Radiant for more energy. If you are concerned more about situational damage you can also focus on the other assassin insignias. Another thing to bear in mind is an assassin should try to do what they need to have 13 Critical Strikes as it provides +3 energy versus the +2 energy at 12 Critical Strikes.

The build I like to use most often in PvE situations so long as there isn't too much enchant removal going on is: Golden Phoenix Strike, Critical Strike, Moebius Strike, Death Blossom, Critical Agility, Critical Defenses, Mystic Regeneration (dervish skill), and Assassin's Remedy.

For the record, this build tends to break the mold that assassins should follow as it can allow assassins to become situational tanks, as all assassins should remember that You're not the tank.

Other skills worh considering depending on what you're doing and how you're going about it is Death's Charge, Aura of Displacement, and Shadow Meld as these are skills designed for moving through the battlefield effortlessly. Skills for more defensive purposes would be skills such as Feigned Neutrality, Shroud of Distress, Critical Defenses, and Flashing Blades (elite) as these are skills that can heal, add armor or block incoming attacks.

I am sure I missing some things from this so if anyone else feels they would like to add to some assassin strategies, then by all means. I am sure I could learn plenty myself. If anyone has any questions or are trying to get started on an assassin, I would be more than happy to help you out as this is my thing [i]