Aaron Marino of alpha m. discusses signs that running shoes need to be replaced and features to look for or avoid when selecting.
Alpha used to wear shoes that weren't designed for running when doing cardio. They had no arch support and had hard Styrofoam bottoms but they were 'cute' (he was a sneaker diva). His body was in pain, so he purchased the most comfortable pair of New Balance. He was in heaven.
In the course of two weeks, his cardio and body was feeling great. But about 4-months later, he started having pains like when he wore shoes that weren't made for running. Running shoes should be replaced within 300 -500 miles, so Aaron's shoes needed to be replaced.
When the outer sole (hard, durable rubber on the bottom) starts to degrade, become soft, and wear-out, the shoe needs to be replaced. When you try shoes in the store, they should be comfortable immediately. When you try shoes on in the store, walk on the tile rather than the carpet to feel the actual cushion. Save your receipt in case you have to return them after your first run.