I track the mileage on each of my pairs of shoes by using a feature available to members on the Running Room website. The feature updates the mileage on my current pair of shoes everytime I enter my daily mileage. I started using this option after I had an IT band injury a few years ago which was partially caused by using a worn out pair of shoes.
While I certainly can feel when a pair of shoes is starting to wear out due to the lack of cushioning in them and more aches in my legs after a run I find that tracking my shoe usage online is ueful in giving me an idea of when I will have to cave in and buy a new pair of shoes.
Most of what I have read has suggested that a pair of shoes needs to be replaced after 700-800 km (430-500 miles) and I find that by the time I hit 700 km it is time for a new pair.
My current pair of shoes has 447 km on them so far. At my current pace I will be looking to replace this pair toward the end of next month.
Do you track the usage on your shoes? How do you decide when it is time for a new pair?