The best way to do it is with a heat gun on a low setting, keeping it back from the plastic about 6 inches, and keep it moving over the area. if you keep it it one spot it will melt, burn, or warp it. As you heat the plastic enough you will see the stress marks blend back in. As long as you're careful to not overheat it you shouldn't have any problems with damaging it or making it brittle. I do not recommend using a torch or anything else with a flame, makes it a hell of a lot easier to burn or melt it, and it doesn't spread the heat over a large area like the heat gun will.
Just be careful about leaving the heat in an area too long or taking it too far. If you smell it burning or see it start to warp at all that's a sign that you've gone too long. You probably won't be able to remove every trace of the stress marks but you should be able to get it at least as good as tyler's looks. Once it gets to the point that it looks like it's not improving stop right there, it won't take long from that point before it melts.
