What to do for a Creased Latex Mask
If you find that your mask has been improperly stored, and looks fine, except for a fold or crease running across it, you can minimize, or remove the crease completely, by placing one hand inside, and warming the folded area with a hair dryer.
If there are multiple folds, and the mask has no hair, I suggest dropping it into the home dryer, set on low, or medium. That will remove most creases.
In both of the above scenarios, allow the mask to cool on a supportive base, or stand.
Here are shots of a badly creased Tor Johnson Mask. Then undented and repainted.