(You need two pokemon on the field to do this trick) when there is a shadow pokemon on the opposing side (eg Tyranitar) use a master ball for the first pokemon's move.
For the next pokemon's move, switch the master ball with another poke ball with the Y or the X button, when the master ball catches the shadow pokemon look in the poke ball collection and the master ball will still be there! (also works with other poke balls).