http://youtu.be/-kNpoo25ywk
Chris Patrick gives a good demo of the standing cable fly. What is particularly good is how he controls the movement in both directions – inwards and outwards.
Apart from the excellent pointers he gives, is it also good to have the bend of the elbow pointing slightly upwards, thus sliding the shoulder blades flat onto the ribs and engaging the pec major. It will feel as if you are drawing your little fingers towards each other. If the shoulders roll forwards, much of the work will come from the troublesome pec minor, the muscle that rolls the shoulders forwards and down.
The exercise can be varied by changing the height of the cable.
It is possible to stand forwards of the cables, so starting to pull from behind the body. This will certainly intensify the workout, but the natural shoulder position has to be excellent to avoid injury.