The first thing to breakdown in a shoe is typically the midsole, even though the upper, and even the outsole, may still be in very good condition. You can go by miles or time in the shoe. That is usually a reliable indicator of when to replace them. 300-400 miles is the standard. Even at 200 miles you can tell a noticeable difference. In a new shoe, same make and model, will feel higher off the ground, have far better support, cushioning and over all the shoe will simply feel a lot better. The best part is though it makes being on your feet all that more pleasurable while at the same time providing the optimal health of your feet and body.