When buying running shoes, consider purchasing two pairs at a time. Here's why: To absorb the impact between the ground and your body, shoe manufacturers use a special material in most running shoes called ethylene vinyl acetate (EVA). When you run, the EVA compacts and requires about 24 hours to regain its shape. So rotating shoes every other run maintains maximum protection against pain and injury.

http://www.menshealth.com/triathlon/run-what-you-need/Choose_the_Right_Gear.php