I have heard of some people - putting it out in the sun (again) to reheat it - then place heavy objects on it to flatten it out.
I've also heard of others who have trimmed the mat down into a usable size (e.g. the portion that isn't warped).
And... I've heard of others who have trimmed it down into circles then placed the circle on top of a lazy suzan to make trimming finished squares easier.
Having said that all - I haven't done any of those.
Good luck.