This post is in response to a question as asked on Yahoo! Answers.
Training your body and ultimately participating in physical exercise will over time lower your heart rate. Average Heart Rate information is available through the main Average Body website, and varies quite dramatically by gender as well as age.
This great resource offers a lot of good information on the subject, though tends to get quite technical. As such, we will attempt to do here at The Average Body what we do best - Summarize!
In a watered-down sense, physical activity excites a number of neural and chemical components within the Human Body. This excitement leads to an increased activity within some of your nerves, namely the nerves that are 80% responsible for the slowing of your Heart Rate. The increased exposure to exercise eventually serves to slow the resting and ambient Heart Rates.