How do I get rid of deodorant stains?

This is an embaring question. How do I get rid of those unsightly stains? In fact, mine aren't discolored like those nasty brown stains you see. These are a funny white color from the deodorant, and the worst shirts have developed a wrinkle or two under there and seem to be getting 'harder'. Luckily this is only on a total of about 5 shirts, and I'm getting rid of the worst shirts. But how about the ones that are starting to change color? And how do I prevent this in the future? Is it too late for any of the ones that I have left which are slowly mutating? Thanks!!