I learned a number of tricks.
1. Use Shoe Goo
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I coat the outside of all my shoes with Shoe Goo in the spots where I wear them down fastest. I do this when they are brand new. It dramatically adds to the life of the shoes, even regular trainers. However...
2. Go minimal
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All though high school and college, I wore normal, thick trainers. When I finally started doing higher mileage, I found I had to switch to LESS shoe in order to keep from being injured so much. The irony is that racing flats are generally (not always be generally) cheaper than trainers. They don't have all the marketable gimmicks. Also you find that when you are not relying on the cushioning of the shoe (because it has none), the shoe lasts a hell of a lot longer. So you get more shoe for less.
If you think you CAN'T go minimal, ignore this. Seriously, I don't care about the subject enough to try to convince people to run like this. All I know is it worked for me and the fringe benefit was spending less money on shoes.
3. Get good at finding deals
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Once you find a shoe you like, scour the internet for coupons, sales, etc. Sometimes you can get last year's version a lot cheaper. Regardless, once you find them on sale, buy in bulk