Lots of good info on here. I do pretty much everything that has already been stated, plus.........sprints.
Only way to mimic the adrenaline rush/physical stress of a monster behind your pin. Ill run a sprint as fast as I can (however far/fast it takes to get your heart rate spiked). Than take the shot. Great way to get in the habit of doing body relaxing exercises (breathing, etc), and its just good exercise.