Shot aim is semi-auto, you can't aim for a top-corner consistently.
Whatever it says in "Training", when you actually play a match, you aim for the top-left corner with the d-pad or left-stick, and the ball goes top-right!
Positioning is as important as aiming in PES3.
If you want spectacular volleys etc, use :L1: + R3 man. pass, or :L1: + :triangle: , for your final pass. This is because your pass will often reach the player out of position.
Sometimes your player will do a sort of sliding/sweeping volley like this, if the ball is ahead & to the side of the player, sometimes a very powerful half-volley. Press :square: just before you want to hit the ball.
For low-ish, driven shots, try laying the ball off when in attack, inside the 18-yard line, back to someone 10 or so yards outside the area (Mutu's goal gallery has a good Gerrard example of this). Then, if you tap the :square: button, your player will really smack the ball hard, quite often towards the top corners. This also applies if the ball runs-free/is deflected outside the area.
My advice is almost always use less power than you'd think.