The normal range is between 60 to 80 beats per minute (BPM). An athlete could be 40 to 60 BPM.
Consult your doctor if your resting heart rate is consistently above 100 BPM.
The best way to improve your resting heart rate is with exercise. Start off slow and work up to interval training, in which you run hard, then slow down, and repeat for a specified amount of time or repetitions, once or twice a week.
Intervals strengthen your heart, allowing it to recovery faster, so your heart rate drops safely after intense exercise and stays lower when you're at rest.
Long endurance workouts are also important. Unlike other muscles in the body, you have to work the heart with longer "reps" and "sets." You might do three sets of 10 reps for your back muscles, for instance, but extend those sets and reps for your heart by doing 1- to 2-minute sprints (or reps) or 5- to 10-minute intervals, for example.
-- Paul Robbins, Metabolic Specialist, Athletes' Performance