Replaced four starters. whats making them go bad.

FIRST things first, did you remove the HEAT SHIELD or is it missing the heat shield..., make sure all of the cables are good, and clean... make sure you put a good quality starter on the vehicle, and make sure you have a good heat shield around it, I use the wrap that they use on race cars... it costs about 20-20$ but is worth it to me, I used to fry starters and have only replaced one in the last 3 years now