I found an interesting document describing how you can slow your heart rate without the use of drugs. Although its purpose is especially for handling tachycardia attacks, some of the tips are quite interesting and may be quite useful for us too. For example the "gentle massaging of the right carotid artery" sounds interesting and I am going to try it before the next training 
The Doctors Book of Home Remedies Tachycardia
The Doctors Book of Home Remedies Tachycardia