The shooting system defiently isnt 100%, but its pretty damn good. Its definetly not about powering the ball from outside the penalty box, hell even the goals I score from outside the penalty box arent scorchers, they are just well placed usually lofted. Even upclose shots just have to be well placed or alternatively you have to use clever passing that keeps the defence and goalie guessing and confused.
But I think the number one thing is touch or power, be very light on the shoot button, lighter than you think you have to be, and more shots will be on target. Unless you have someone like Henry who's shot power and accuracy are about the same, you'll have to hold back, example player Babel or even Fabregas I think, whos power generally far outstrips there accuracy.
I personally find a powerful shot is maybe a little over a quarter of the power bar, and I alaways aim to keep it under half.
Someone also made a good point about wether you are running, or you have pressure on you, or the angle, or a weak foot shot, or things like transitions where turning and immediatly shooting without some settling of the player decreases accuracy. Also if the ball is even a little bit up in the air that just means you will need less power...really all those things are as in real life.