Originally Posted by DrWJODonnell

Primarily long saddle hours. Though all training helps. Lower heartrate indicates more pump (heart/blood) efficiency. Some are naturally low (mine was 50 prior to lowering it to 38) but typically years of training and adaptation to the stresses placed upon it lower the HR.

I would not worry about getting a freakishly low heartrate as it does little to tell you of your race performance. However, if you are using HR training and your resting HR is still upwards of 80 (and you are not a child) you may need to rethink your training program. Overtraining can elevate the resting HR (though in most cases not by more than 10-15%.)