It is impossible to prevent if you dont know what your doing. I have asked many people after this happening if they have ever had their rims chipped and I have yet to come across someone who has told me yes. He had this brand new machine that had rubber over everything so that it would not scratch rims and ranted about how he has installed a 300 rear tire on rims without scratching them.

Create a Poll about how many people have had their tires changed and ended up with a scratched or chipped rim and you will see the outcome. I may be mistaken but I have not met anyone to date who has had this experience.