I start with a pair of good insoles, then as those wear down I buy another, and another, and so on until the shoe tread wears out or holes appear in them. If you buy some of those magic eraser pads, you can usually keep the outside of the shoes looking relatively new for a lot longer as well. I have some shoes right now that are going on ~1.5 years and I'll probably get another 1.5 out of them.

Pro-tip: don't buy the cheapest shoes you can if you can afford it. I used to buy $20 skate shoes and they would wear out in less than a year.