Bob Wright has it correct. Concentrate on the sights and squeeze the trigger.
Concentrating on the sights keeps you from anticipating the shot by giving the mind something else to think about. And the squeeze lets the shot come as a surprise. At some point, you will forget about the squeeze and know that the shot is going to go off because you make it go off. At that point, you are well on your way to mastering the art of shooting well.