I have had luck reshaping masks using a heat gun to lightly and carfully heat up the offending area and the let it cool in the correct position. I have also heated masks by placing them in a dryer to soften them up. I have never tried this on an exspensive mask....just the budget variety.
Matthew, I did an article in my blog on this subject recently and showed some before an after photos of a mask that was pretty bent out of shape to start with. You can check it out at Halloween Asylum News