I've always had to throw away tennis shoes when the right foot sole gets worn out. But with my current shoes (New Balance), I noticed that the soles are holding up, but instead both insoles are worn out have multiple holes - areas below the big toes and hills. I'm wondering if one of the two results is more desirable - the sole or the insole to be worn out first? With my current shoes, I'm guessing that my feet are slipping inside the shoes more than with my previous tennis shoes. Is this something to be avoided? If so, what kind of insoles are good at preventing such slippage and still provide ample cushioning? I've used "SuperFeet" insoles that CitySport sales people strongly recommended, but they had little cushioning and were hard on my feet.