It's difficult for a pre-60 thief in a non-static situation. There are optimal circumstances, there are good circumstances, and there are poor circumstances.
The worst thing you can do is use your partner. If they're a dragoon this is fine if your tank is handling themselves well. Realistically speaking, in a mid-level pickup party, sometimes this is the only realistic option. But it really sucks when you set yourself up as the antagonist to the tank, and I've killed lesser dark knights by doing this. On the whole not fun at all.
The next best option is to split your trick attack away. You won't miss a ta/ws any more than any other meleer will miss theirs. You don't have to worry about the mob turning from the tank then. You can prime a SATA, use that at the start of the fight, then when the skillchain comes around, you have a TA/WS and then a followup SA by yourself. Not the most optimal situation before assassin, but it works. If you have equipment sets boosting TA and SA independently, it works even better.
The best option is to try to get your skillchain partner to turn the mob from the tank. This is a ***** to learn to do. It requires paladins to back off for one, which is often an insult to them. It often requires you to stop priming SATAs so that the tank doesn't have 350 damage worth of hate at the start. Unless they're a ranger, your partner almost always has to sub warrior, and provoke right before using their weaponskill.
Once you get assassin, things lighten up. Splitting your sneak attack away from your trick attack starts to do more total damage than stacking them. If hate is sufficient, which it should be, you will only have to ta/ws the tank. I would set up a macro pad so that you are capable of handling any situation. When the mob doesn't turn, hit your TA equipment macro, your TA skill macro, and your weaponskill. When it does, hit your SATA equipment macro, your SATA skill macro, and your weaponskill.
Until you get to 60, though, just do what's practical while hoping for the optimal. You're not going to get it often. If you're having a rough time even landing something, be anal about the line. You have to be. Be anal about the tank being right up in the mob's face, and be anal about the partner riding the mob's rear. If the mob starts walking when this is going on, it's over. You just blew 130 tp and have nothing to show for it.
Number one advice I could give a thief is that there has been a tactical shift away from insisting that SATA be kept together. Be prepared to split them. Never blow a sneak attack because the mob is facing the wrong way--macros can avoid that, and attention can avoid that.
. Anyways, you need to learn that and be ready to run to the other side if it doesnt get turned.. That is mainly the only thing you NEED to do in a party situation, aside from pulling when its asked of you. If you cant get it right, then you shouldnt be a Thief, sorry but those are the facts...
