It always surprises me when an injured athlete comes into the office with excessively worn running shoes and doesn’t know how old they are. Were the shoes purchased six months ago, a year ago? Has the athlete been wearing the shoes for more than a year?
Even though many factors contribute to wear, I always instruct my patients to write the purchase date of their shoes directly on the shoe insole underneath the sock liner to at least give them an awareness of shoe age and wear.
I then tell them to check for signs of wear every month starting at month six using the following YouTube video I produced as a guide:
http://www.youtube.com/watch?feature=player_detailpage&list=PL8FBD9F2021569064&v=ylgzgbzC9i4 .
Although excessive shoe wear can still occur, it is less likely to occur when using the above methods as opposed to simply replacing the shoes after an arbitrary 300 or 500 miles.
Dr. Jenny Sanders Shoe Blog 