How do I make a halloween mask look right?

I think it's vinyl. ...or plastic. its one of those ones that fits over your head and it smells weird and I got it with the costume. it's actually a Jack Skelington one. U know, from Nightmare before christmas) =) Anyway, I tried putting a balloon in it, and it seemed to have worked, but I want it to look spherical, like in the picture. But there's still some fold lines and dents. what can I do to fix it?How do I make a halloween mask look right?
try and seal the dents with glue if it is a white mask, i tried it once!How do I make a halloween mask look right?
I think if you duct tape cardboard inside of the mask that might work. But I have never tried it so it might not do the job.