Golden Rule of Foot:Once a month place your running shoes on a flat surface and look at the heels from behind. You will be able to notice if one side or both sides are beginning to lean inward or outward (as illustrated in the photo above). When this occurs, it is important to get a new pair of shoes immediately. I think that this is good practice when buying athletic shoes in general. A shoe should always feel stable, but it must look stable or centered from this back of the heel view.
You can see from the photo above that the right side is leaned to the outside more than the left. This is called excessive supination or lateral instability.
