stripped stud problems

[Carrasco]

Strange title I know, but it's the best way to describe my problem. One of the studs that holds my exhaust manifold to the head came out today, tried to put it back in, but the threads in the head are stripped - and this is the 2nd time this has happened with a different exhaust/head stud. I just paid a lot of money to get the other one retapped. This is on a 92 celica GT with a 5F-SE engine and aftermarket garrett turbo from Burien Toyota. I'm not sure if this is a corrosion, heat, or something else, but what ever it is, I need to prevent it from happening again if I can. Any help/suggestions/experience would be great. Thanks

James