From The Federalist:

while a lot of ammunition is needed for serious practice over time, the most efficient way to improve one’s firearm skill—in terms of the time, money, and energy invested in relation to the return on that investment—is without ammunition, by what is commonly referred to as “dry firing,” the required safety precautions for which can be explained by any competent instructor.

According to former Army Special Forces soldier Mike Green, whose company, Green-Ops, conducts defensive firearm classes and dry-fire clinics in Northern Virginia and South-Central Texas, “dry-firing is the most often overlooked element of a comprehensive training program. But it shouldn’t be. It’s simple and almost cost-free.”