Face it - athletic shoes wear out even when they are not used every day.  They may still look pretty decent and even feel comfortable but may not be providing support you foot needs for exercise.  The investment in new shoes will be worth it if it helps to prevent aches, pains, and even more serious injury. 

How do you know when it is time to replace your athletic shoes?  Read this article by the American Academy of Podiatric Sports Medicine for some great tips.